It all depends on your priorities. I have traveled extensively in Nicaragua and have not yet been to the Corn Islands because there is too much of interest on the mainland. The Corn Islands look great and are indeed were you would go to snorkel. I would prioritize other things but it is not my trip. If you do go to the Corn Islands you will need the day you fly to Big Corn and maybe take the boat to Little Corn and I assume if you are going to go all the way there you will want 3 full days there and then you would have to fly back to Managua. Are your 12 days actually 12 full days or 11 days or even only 10? If you have 12 full days you will have 7 days left after the Corn Islands. That is enough for Granada and Leon. I suggest though you go to Ometepe Island as well. The economy is based on livestock, agriculture and tourism. You might find this interesting:
https://retirenicaragua.wordpress.com/tag/cattle/
You could do a homestay:
http://www.losramos.org/
http://www.puestadelsol.org/index.php
Ometepe also has two volcanoes (tough climbs), lake beaches, a nature reserve with a nature trail where you can see monkeys and birds (especially at dawn and dusk), a waterfall, a natural spring pool (Ojo de Agua), kayaking... You might see wildlife at the waterfall, at Ojo de Agua, on the volcanoes...

If you want to see wildlife you should really go to Costa Rica (we saw so many sloths there and two tapirs and a puma with two cubs but seeing a jaguar is almost impossible... most locals have never even seen one) but one of the best spots in Nicaragua is the Rio San Juan and I do love Los Guatuzos. You can fly from Managua or Ometepe to San Carlos (but I believe only on Thursdays and Sundays) or you can take the bus (6-7 hours) from Managua. The boat to Los Guatuzos goes most days but not daily and takes 4 hours. You will want 2 days at Los Guatuzos. Check the centro ecologico Los Guatuzos and Cabanas Caiman. Wetlands, rainforest, lots of wildlife. We saw lots of monkeys, birds, caiman, giant iguanas, lizards, bats, red-eyed tree frogs and a sloth... We also went to El Castillo and I hear a lot lately about Basecamp Bartola but I have not been.

So, you might want to choose: the Rio San Juan or the Corn Islands. Or if you skipped both you could spend 3-4 days on Ometepe, 4 days in/near Granada (don't miss active volcano Masaya), 4-5 days Leon if you want to do Cerro Negro and the overnight volcano Telica trip and go to Las Penitas for the beach.

Another good spot for wildlife is not too far from Granada and Masaya: Chocoyero-El Brujo. Waterfalls, trails in the dry tropical forest and we saw lots of birds and monkeys and also (lucky us) an anteater and a boa constrictor. You might also see wildlife on volcano Mombacho near Granada (maybe even a sloth) and near Laguna de Apoyo (we also saw lots of butterflies there).

I have not been in May but it is the start of the rainy season so it could still be pretty dry or you could get short but heavy showers, mostly in the afternoon and evening and at night. Some rain would be nice actually to cool things down and to make it a bit greener as it will have been very hot and dry. You should probably expect a mix of sun and showers on the Corn Islands.

You certainly have more than enough time to visit all the National Parks and key areas. You might want to add Panama or Guatemala, if hiking is a focus as well.

FYI- English is widely spoken in CR. The other countries not so much, though most popular tourist areas english is fine.

What time of year is this trip? Weather can be a major factor, especially on caribe side and also Osa, two of the most intense and bio diverse areas of the country. I would avoid CR in October.

If you will be using public transport, then time is a nice thing to have plenty of, as not all the areas are easy to access between eachother, and require more connections and going back thru the capital.

You also can use shuttle services between many areas, as well rent a car, and take it oneway and drop it as well. Some areas you dont want a car or can even drive one, other areas are wonderful to explore in a rental and nice driving.

I suggest you get your hands two books, National Geographic Explorer Costa Rica, which covers all the important parks, landscapes, eco systems and bio diversity.

Then get the MOON Guidebook on CR, same author Christopher Baker. This book goes into much greater detail and is also more of a bible for getting off the beaten path and to areas few people access or have time for.

Costa Rica has 3 main areas, plus beaches.

The Caribe side is a important focus as its mainly rainforest and jungle, and has the most bio diversity and tropical landscape in the country besies the Osa-Southern Zone and the key park in country and region, Corcovado.

The caribe side areas that are a must, are as follows-
Sarapiqui-La Selva- This is a international private research park...30 miles of groomed trails.
Tortuguero Rainforest and canals- Hike, kayak, boat.
Cahuita NP Hike/Snorkel/Swim
Manzanillo-Ganduca Marine refuge Hike/Horseback/kayak/swim
Rafting the Rio Pacuare. Through rainforest, jungle, waterfalls and Indigenous Indian lands.

The other area is the NW area, which is also best to discover via a rental, but not a must, just requires more planning. This area includes:
Monteverde - Hike
Arenal Volcano- Hike, hot springs
Tenerio/Rio Celeste- waterfalls/hike/swim
Cano Negro- boat/fish wetlands/everglades/rivers
Rincon Hike/waterfalls

Between the NW and Caribe is two great hiking areas-
Orosi Valley- Pine forest and trout fishing
Gerrado de Dota- Hiking/Hummingbirds

Tallest Peak in CR is Chirrpo in La Amstad- 2 days 1 night climb 11,500 ft
La Amistad/Talamanca also cloudbridge to hike and thermales at base.

The Osa/Southern Zone- Key park here is Corcovado, one of the most bio diverse parks on the planet. Hiking park and sleeping at ranger stations requires guide/permits/reservations. Day 1 22Km in raw jungle, day 2 18 Km along coast, requires river crossing times w/tides.

Can access Ranger Station via boats from Drake Bay as well, but not connected to base for hiking in Pt Jimenez.

Other key areas to explore in Osa and Southern Zone is San Vito to San Isidro de General/La Amistad area.

Nicaragua is easy peasy from CR vis main road/buses from Liberia to border via Pan Hwy/CA1, and or via bridge or boat from Los Chiles to San Carlos. Things have calmed down there now and civil unrest has been oppressed, but could flare up. Businesses still closed catering to tourist, so they need tourist more than ever...Key areas to explore there are Ometepe, Leon, Granada and the north near Estelli.

Panama you can access via two border crossings, Caribe side at Sixoala to Bocas Del Toro islands, or via the Pan Am CA1 Hwy on pacific side, to David/Boquette, Volcan/Baru area for great hiking too..and La Amistad as well.

Guatemala has a lot of culture and volcanoes to hike, great landscapes and ruins/mayan culture. If it were me, 3 weeks Costa Rica, 3 weeks Guatemala..

Have Fun!

Yes, you would fly to the Corn Islands from Managua and then back to Managua. From Managua you can go to Leon, then to Granada and volcano Masaya, Ometepe and SJDS. Or skip SJDS and spend time in Las Penitas near Leon. In that case you could go to Ometepe, Granada and volcano Masaya first and then Leon and Las Penitas.

Ojo de Agua is probably the closest to a "jungle pool" and that is on Ometepe. You might also enjoy Laguna de Apoyo near Granada (a crater lake) but I don't think you have time.

San Juan del Sur: if you like to party you will probably love it. But yes, people do get drunk of course. For me the hardest part was finding a budget hotel that was quiet enough to sleep.

We have often been lucky with festivals in Masaya. I believe that on some or all Sundays in November they have the Festival of San Jeronimo but you would have to ask when you are there.

Food:
You often get gallo pinto for breakfast. For lunch or dinner we often got a plate of rice, beans, some kind of meat, plantain, cabbage salad, maybe a piece of cheese... And the beans can vary greatly, some are really soupy which we often didn't like as much but some were just amazing. We had no problem finding local food. It is all we ate. In Granada we like El Gordito in the central park. I tried vigoron there (boiled yuca is mixed with crunchy pork rinds and covered with a spicy cabbage slaw). We ate at a fritanga on a side street of La Calzada and on La Calzada there is a small place on the right just before you come to the church. No sign. We found other places as well just wandering the streets.

On Ometepe there was a great fritanga in Moyogalpa (just follow the smoke!). And in Balgue we had a few great meals in a small restaurant on the lake where they chased the dogs away but not the pig! In Leon we liked the street food behind the cathedral and we also found a few great local places not too far from the cathedral. In Las Penitas when you step out of hotel Oasis go right and just before the street turns there is a place with a large patio (no sign) with great local food and sometimes they have cheap and delicious seafood soup.

Oh, and they might ask if you want platanos maduros (fried sweet plantains) or tajadas de platano verde (fried green plantains, more like plantain chips). And then there are tostones, fried plantain slices which we got often and were not my favorite...

Masaya has small local eateries in the central park and there is a great fritanga not too far from Hotel Regis. And we walked to the Tiangue de Monimbo in Monimbo (an indigenous neighbourhood in Masaya). Excellent street food.

Thank you so much for responding to my post! I appreciate knowing where you stayed, what you did, how much it cost, and your reassurance that if we aren't staying very long in Nicaragua, that we could possibly not go to Ometepe not regret our decision. We decided to go with our gut feeling that we didn't want to get stuck on an island with limited transportation during Holy Week, and instead are going to Somoto Canyon and on a tour near Esteli. We will have to pay extra money for private transportation on those days because apparently there will be no public transportation on their national holidays of Thurs/Friday of Holy Week. I am very grateful for Whatsapp, with which I was able to contact the tour operators and the hostel in Esteli where we will be staying. If it were any other time of year, I would feel comfortable with finding accommodation once we arrive, but with our constraints and family size, I feel at ease knowing that we have a few days planned out.

I can't comment on the Nicaragua portion as that is mainly a trip based on your interest, but it seems like a lot of moving around to me...you will be go go go...

As for the logistics of getting from Ometepe/Rivas to Liberia, there is no need to take the Tica bus, as that schedule may not fit your needs.

From Ometepe to San Jorge, the water taxi's take about an hour. From San Jorge, taxi hire to Costa Rica Border. Croos border on foot/immigration/customs and be in CR in about hour depending on lines.

Buses depart the Penas Blancas border every 30 minutes in daylight to Liberia, about 2.5 hour ride. Once in Liberia, you can then either get a shuttle from there to La Fortuna, about 3 hours, from a local hotel, or head to the main intersection of the Pan Am CA1 and the turn off to LIR airport (Burger King/Kentucky Fried Chicken plaza) and meet the shuttles there, though they mainly make this run in the morning. Check Interbus/Grayline schedule.

Otherwise, you would get a bus in Liberia south to Canas, then get off and switch bus to Tileran, then get a us to La Fortuna. Not sure if there is a bus from Canas to La Fortuna, or a connection. That would take at least 5 hours though. Same route as the shuttle of if driving, just public transport.

That all said, Arenal Volcano is not active. Though you can enjoy the Hot Springs and cooler mountain temps up at Arenal and hike the park at the base.

You could also rent a car in Liberia, and drop it at SJO, for $55 day with insurance...a shuttle for 2 people from Liberia to Arenal will be $55PP.

Also, you will pay for the full trip on Tica Bus, from Managua to San Jose, even though you are getting it in Rivas and getting off in Liberia, which is 1/3 of the normal trip distance...

There is no direct bus...

So either go back to SJ for a direct bus to Nicaragua Border, or to Managua with Tica Bus or King Quality. Its 6 hours from SJ to Border...

Otherwise, Quepos to Puntarenas, 2 hours, then Puntarenas to Liberia, about 2 hours, Then every 30 minutes in daylight there are buses from Liberia to Border, about 2 hours...long day no matter what.

Once across border, taxi hire to SJDS or to San Jorge for ferry to Ometepe...