Just so you know, the focus on wildlife should not really focus on or around Arenal, Monteverde or Manual Antonio, those are just very popular tourist destinations the tour companies go to-
I suspect the above posters are in the tourism business their answers warrant concern.
Also, Liberia is best for visiting the pacific surf/swim beaches, and visiting the Central/NW areas, but far easier with a rental car, not by bus/shuttle between the points you mention, that will be more costly more time consuming, and not your schedule.
I would also consider Cano Negro, Tenerio/Rio Celeste and Rincon, for bio diversity, nature and great landscapes, few tourist buses and ho hum beaten trail.
Arenal Volcano is not even active. Lots to do around there, but not really wildlife/bio diversity compared to other areas (Sarapiqui is excellent).
Monteverde is a Clouforest, and at that time of year, can still be very wet, misty, foggy and not really as rich in Bio Diversity and Animal life, as other areas, like Sarapiqui and Corcovado-Osa. It is also very touristy, and very out of the way from anything.
Manual Antonio is more like a outdoor mall, its a small park, throngs of people and noise, small beach and very touristy too...
Corcovado is the gem of the country and region, but it also depends what you want to do there, in regards to where to stay location wise. If you plan to hike the LOOP (Guides/Permits/Reservations Required) you need to base in Pt Jimenez, which is best. The hike 21Km day 1 and sleep at Sirena Ranger Station, and 17Km day 2 and exit out La Leona/Carate side.
If you wish to access the park as a day trip, then Drake Bay is best, and your lodge arranges everything and you go via boat, which can also be a overnight at the San Pedrillo or Sirena ranger stations. Reservations for lodging is best, and allow at least 3-4 nights in the Osa-Corcovado area. You can fly direct from San Jose, but not Liberia.
The other area of the country, which is rather far too, for Liberia, is the caribe side, as Sarapiqui, Tortuguero, Cahuita and beaches south of Pt Jimenez, to Manzanillo, is all jungle, rainforest and far less tourist, far more rustic, and even cheaper too. You also could raft the Rio Pacuare, a Top 10 river in the America...
There is a lot more to CR than the BIG 3 and Zip Lines, been going to CR since 2001, 35+ trips, never had need to go back to Monteverde, and Arenal is not active. The caribe side on the other hand, and Osa, all the time, best part of the country, as well as Tenerio/Rincon for hiking.
Get the MOON, LP or Roughguide books. Don't rely on the internet for where to go, its just the same same circus of tours...
