Can vouch for the Arenal Observatory Lodge. ^ Went in 2005 (I believe --lots of trips since then!).

The Lodge was fantastic and we had a balcony looking right out at the beautiful volcano, and also lots of birding action right from our room and howler monkeys to boot. We were lucky and had nice clear weather and great views of the volcano.

We only spent two nights there before moving on, but we are definitely going back when we make another trip to Costa Rica. Still lots of areas we didn't see.