I'm happy to share my experience from a March visit to CR. I spent 10 days driving around in a rented 4Runner. My favorite place for wildlife was Manuel Antonio, a small but very popular park on the Pacific Coast. I stayed in Quepos so I could be out at first light. It felt like I had the place to myself for several hours before the crowds started to show up. I encountered sloths, howler monkeys, groups of capuchin monkeys, beach crabs, roosting bats, large iguanas of various colors, a large snake coiled up on a low hanging tree branch and a variety of large birds. (My hotel hooked me up with a local guide who helped me spot animals.) I almost didn't go because of its well deserved reputation for being crowded. I'm glad I did. It was a great location.
On the way to Manual Antonio you pass two other worthy stops for wildlife viewing. The first is the Rio Tarcoles. There are huge crocs along its banks. Pairs of scarlet macaws can be seen at sunset flying back to their roosts for the night from the highway bridge that crosses over it. Not long after crossing the Tarcoles, you get to Carara reserve. Several worthwhile trails run through it. I saw toucans, hummingbirds, anhingas, streams of leaf cutter ants, coatamundi and lots of other interesting flora and fauna. The best trail followed along the edge of the water. Not sure if it was part of the Tarcoles or something else. There were caiman (or juvenile crocs) in abundance and basilisks too (Jesus Christ lizards that run across the surface of the water). I stayed at a nearby Marriott resort (Puntarenas) so I could be out again at dawn. I had trouble handling the heat and humidity once it started to approach noon.
I visited other areas of the country too but they were not as good as that stretch of the Pacific Coast. The Monteverde Cloud Forest was much cooler, a welcome relief, but it lacked the variety of wildlife found at lower elevations. Most folks travel to Monteverde to see quetzels. I didn't see any but I did enjoy seeing an amazing number and variety of hummingbirds. The Arenal volcano is relatively close to Monteverde. It erupted the night I stayed there. Unfortunately, the eruption occurred on the opposite side of the volcano. Never occurred to me that this could be a problem. :-(
One place I didn't get to was the Osa Peninsula. I would have liked to. Seeing a jaguar anywhere is a long shot but your best chance would be here. This is probably the premiere place to view wildlife in CR. It is also hard to access and even more hot and humid than the places I did visit. If I were to go back to CR, I would probably fly to the Osa Peninsula and spend at least a week in just this one area.