Hi Caon,
My wife and I have been to CR three times in the last fourteen months (we are gearing up to winter there, perhaps). Trips have been two weeks, 12 days, and 3 weeks in duration. We find staying three or four nights in a location is a good compromise to experience a location and minimize travel days. We prefer to fly in and out of Liberia.
During high season, it is well worth booking in advance.
We did Carara in September. Be prepared for hot and humid. Cerro Lodge is a good place to stay.
Near La Selva, we stayed at Chilamate Rainforest Eco-Retreat. Quite nice, and an enthusiastic young birder leads morning birding/nature walks. It is about 15 mins drive from La Selva and 20 mins from Tirimbina Reserva.
Monteverde has (so far) been our favorite. Monteverde Cloud Forest Reserve, Santa Elena Cloud Forest Reserve and Curi-Cancha Reserve offer similar, yet different experiences. Curi-Cancha is perhaps best for birding. If you can, book a room at Casa Batsu. Reasonably priced, awesome breakfast, and walking trails on-site. Carlos and Paula are excellent hosts (and have become good friends of ours). Be sure to take Carlos up on one of his dinners while there. We also stayed at Hummingbird House (via Air-B-n-B), which was nice because we could cook for ourselves, and was a couple hundred yards from Bosque del Tigre and easy walking distance to Curi-Cancha, the heredia (ice cream), Monteverde Coffee House and Stella's Bakery.
You might also consider Arenal. Our only experience here is at Arenal Observatory Lodge, but that was quite good - we've been there twice.
If you are interested in a northern Pacific beach experience, consider Lagarta Lodge, near Nosara. Excellent birding on their adjacent private reserva, and the morning boat tour on the river is very good.
Enjoy!
Dan