Easily one of the most biologically diverse parts of the country is the Osa Peninsula which is home to the Corcovado National Park. The perfect destination for naturalists, a number of eco-resorts have popped up in the last decade the area is still very underdeveloped and a great place to get back in touch with nature. It should be noted, that although you may find a surprising shortage of pesky insects in other parts on the country, that's not the case here. Bring along your mosquito repellent.
Surprisingly, Puerto Jiminez is the biggest town the area, and there's not much to it, but it's the last civilization before you head of into nothing but nature.
The best in Puerto Jiminez is the Iguana Lodge where you'll find lush rooms, good food and help arranging any tours or excursions you want to undertake while in the area.