I would no more spend time in SJDS than in a trash heap.

Spend more time on Ometepe...

But from ferry at San Jorge, taxi hire to CR Border..

Then once across border, get a bus (every 30 minutes) to Liberia, about 2 hours or more. Then from there a bus to SJ, another 4 hours, and get off at SJO/Alajuela...before it goes downtown, unless you wan to spend the night in the capital..but its $28 and 45 minutes back to SJO next morning...

I don't dive but there are indeed better places for diving than Costa Rica. And will you have time to travel in CR till the end of March? There is of course lots to do right in CR. You could go to Nicaragua in April. You could easily spend a month there: Ometepe, Granada with volcanoes Masaya and Mombacho, Laguna de Apoyo, Chocoyero-El Brujo, Leon with volcanoes Cerro Negro and Telica, the El Hoyo trek, Las Penitas and Jiquilillo for the beach and volcano Cosiguina. The northern highlands: Esteli for the Somoto Canyon, El Tisey, Miraflor and Matagalpa/Selva Negra/Jinotega. And there is the Rio San Juan: Los Guatuzos, El Castillo, even all the way to San Juan de Nicaragua. And then cross the border at Las Tablillas (Los Chiles). A month would not be enough.

I can't help with going to Curacao for a week but you can easily spend June in Guatemala or in Guatemala and Belize or Honduras for diving. In Guatemala highlights are Antigua and the overnight volcano Acatenango hike, Lake Atitlan, Xela (volcanoes, hot springs, villages...), Nebaj for hiking, Lanquin and Semuc Champey (stepped pools) and of course Tikal...

The only factor of issues to me is the time of year. Oct-Dec is hurricane season in the Gulf/Atlantic, and rainy season too...

Unless I am mistaken, Dec is the rainy time in Bay Islands (micro climates on caribe side)...vs mainland...

Just expect delays dues to landslides, flooding and wash outs.

Spending a month in Guatemala would not be unusual...encouraged :)

Nicaragua- Leon, Ometpe, Granada and the Rio San Juan/El Castillo

Costa Rica- endless National Parks and hiking w/wildlife/nature and bio diversity the focus.

Tortuguero
Sarapiqui
Cahuita
Raft Rio Pacaure
Rincon
Cano Nego
and the gem, Corcovado in the Osa.

Great beaches and surf towns on both coast. Dec 15 to Jan 15 things get nuts in CR due to influx of Xmas break sun seekers...

Skip SJDS, your going to CI and Ometepe. If anything, take in Leon culture and the beaches near there, far better scene...

What about Rio San Juan via San Carlos head to el castillo areas?

You should consider Nicaragua, saw your Mexico Post as well. It's much cheaper overall for what your looking for.

Jiquilillo has very nice surf, it's quiet, remote and few people. Good place to relax, surf, walk the endless beach etc. just a few lodging and restaurant options, no real town.
http://rancho-esperanza.com/ Fun kayaking in Padre Ramos Reserve just north.

A little south is the small beach town of Las Peñitas, a little more going on and good surf.
Both are close to the colonial town of Leon, very cool town. Lots of Volcano activities, boarding, hiking, overnight camping for the lava etc.

The Island of Ometepe is a great 3-4 night place, a variety of activities from mopeds to volcano climbing.
Just an option for you.

Thanks for your answer. Since we are both 25 and do wanna party a bit we are thinking of spending one night less in Little Corn and SJDS so we can spend an extra night in Ometepe and Leon.

I think you will want three full days for all those day trips so leave Granada Friday morning.

I have always heard Playa Marsella is one of the best beaches for swimming. I have heard El Coco can be good too and it a nice beach but then others say it was rough when they were there.

Have you considered Ometepe Island? If you like lake beaches it might be a good alternative. Great swimming at Charco Verde. There is also Playa Santo Domingo. And Ometepe is a beautiful dramatic island with two volcanoes and rural communities. Charco Verde is actually a nature reserve and has a great nature trail where you can see wildlife (monkeys, birds... best at dawn or dusk). There is also a butterfly place. Next to Charco Verde there is Chico Largo Adventure: fun ziplining with great views. And then there is Ojo de Agua, a natural spring pool. There is the San Ramon waterfall. You can go kayaking. Horseback riding...

How basic is basic (what is your budget, how much per night)?

Correct. The flights to/from Ometepe/San Carlos only go on Thursdays and Sundays. And you do need to book them as soon as you know your travel plans. Those little airplanes don't have a lot of seats, and often sell out well in advance.

Or should I skip Solentiname Islands altogether and upon return from Corn Islands dedicate more time to Leon and north west area?

Depends on you. Leon is an authentic colonial city, historically the "leftist" city in Nicaragua, with Granada being the more conservative city. From Leon, you can go to Las Penitas beach for big Pacific surfing waves in 1/2 hours time. And the volcanoes near Leon are completely different from what you find on Ometepe, Volcan Telica being the best one for a quick visit. But of course noneof these places are as serene and natural as the Solentiname Islands.

Maybe it just depends on the timing of the Thursday/Sunday flights.

Its pretty much the same. But keep in mind getting between the Pacific beaches outside of SJDS is area is not as easy as it appears...like CR, you may have to back inland and return to get another coastal road.

There ferry for vehicles to Ometepe from San Jorge is rather small, not like Tambor/Puntarenas CR, so get there early to get a spot, a few delivery trucks can take up half the space on the ferry.

You might want to consider first going to Granada area via taxi hire and using that as a base for 3-4 days for day trips and taxi hire/tours, then get a car in Granada and head out to Ometepe...

The southern areas near Rivas/SJDS is where the cops look for gringos in rentals...

Have fun...