We just came back from Costa Rica, amazing! My only regret: a week is way to short, we are already planning our next visit.
We wanted to see a bit of everything, so we spent 2 days at the Arenal Volcano (stayed at the hotel Los Lagos, great views of the lava, pools, etc. Clean and recommended), then 2 days at the Monteverde cloud forest (stayed at the Sapo Dorado, nice mountain cabins, all wood, rec.), and finally two days of R&R at Manuel Antonio (stayed at Verde Mar, beach front), and finally, one night near the airport. We got to see 3 very different ecosystems.
To plan our trip, we used the excellent Lonely Planet Costa Rica guide book, and the web site of monteverde: www.monteverdeinfo.com You can find a nice photo gallery of the Arenal Volcano at: www.arenal.net
I would suggest to bring good walking shoes, binoculars, and a warm jacket: San Jose & Monteverde were surprisingly cool at night, a flashlight (we got 2 power failures) and finally, take 2 weeks if you can!
Costa Ricans were friendly and very helpful, can't wait to go back...