How much time do you have?
What is your travel style?
What is your budget like?
Will you rent a vehicle?
Costa Rica has more variety of wildlife in some square miles than all of Canada. So if you find a place to focus on, you need not travel far to experience it.. Unfortunately Liberia and Tortuguero will not work unless you want to do a long day's river trip (not on most tourist agendas) or long bus trip across the entire country (7 hours) followed by late day boat ride into a very different kind of place. If you get there, hire Castor Hiunter to take you on a mangrove tour for two in his paddled canoe. He is a kind and informed man. There are several AI resorts in this area, all geared to 2 to 4 day stays. They are relatively expensive. There are many local tourist hotels, but make sure you inspect the rooms as they vary widely from the pictures.
Out of Liberia the biggest bang for your buck as far as flora and fauna would probably be to get up to Santa Elena in the MonteVerde area. The AI resorts up there are probably very expensive. We found a nice, uniquely furnished and appointed local place called the Rustic Lodge there and enjoyed being swarmed by hummingbirds a few steps from our door. The owner was also very attentive and a great source of suggestions for interesting places to see. Good local food can be had at tourist type eateries for about $50 a day for a couple up there and in most places, including Arenal. Likewise, you should be able to find clean, quiet and safe places to stay for this amount.
Edited: 12:22 am, January 04, 2013