Dantica lodge is amazing, I remember falling asleep on the sofa in front of the floor to ceiling windows, watching a thunder storm roll in over the mountains, log fire burning....it's pricey even for CR but a little piece of heaven. The owner is a birder as well so can point out some good local spots. Saw several quetzal the next morning without even trying; the trail at the bottom of the valley (to a pretty waterfall) is very productive. Plus you can add nearby Cerro Muerte for paramo endemics. Another site worth trying for quetzals is 2/3 of the way up Irazu volcano (worth doing anyway) where you go through open oak savanna. Scarlet Macaw is common around the Pacific coast (e.g. Carara NP, Osa peninsula).
There's a lovely lodge just west of Piedras Blancas called Suital Lodge; has it's own little reserve full of birds and a nice private beach & kayaks. Another spot nearby that's really nice is the Las Cruces biological station (San Vito) where they do active research on rainforest ecology & restoration - decent accommodation too and a nice reserve at mid-altitude.
If you want sloths, I can highly recommend the sloth sanctuary just north of Cahuita (Caribbean coast). They do a great tour and have baby ones you can see close up. Amazing beaches and good birds a bit further south around Manzanillo.
But the whole country is a tourist playground in the best possible way, so you'll have a great time wherever you go! Bus is doable but car is way better, and not difficult. San Jose can be a bit full-on during rush hour so try and avoid it