I was in Costa Rica in November. Arenal is amazing - you might be staying there too long though. My recommendation is rent a car and move around. There is so much to do and see!
Our favorite zip lines were in the mountains about 1.5 hours from Arenal. If you're interested, i'll get the name for you.
The food at the local 'sodas' is incredible. Skip the touristy spots for the small, open little cafes. You won't be disappointed.
If you plan on going near Tambor, let me know. I'll put you in touch with a local who's family does tours. You'll get to skip the crowds and see parts of the area that locals know about. We spent a couple of days with him and saw the normal stuff (jungles, monkeys, etc). He also took us diving for oysters and prepared them for us and went to a deserted beach near Malpais.
Some day i'll get some photos posted and give everyone a link!
Lynn