You don't mention if you'll be in cities or in the rainforest, mountains, etc., but I always have insect repellent, binoculars, xtra batteries/memory cards for my camera along with a flashlight.

I usually don't carry a guidebook since I'll have all my reservations and tours planned out in advance, but I do carry some bird or wildlife ID cards. I generally scan and print specfic pages of a birding guide rather than bring it.

Hope this helps!

www.moon.com lots of great CA info.A money belt insect repellant and a good medical kit always wise.Weather2travel.com winter best for me far less rain.
insurmytrip.com many dodgy areas.

sunscreen and insect repellent, of course.

Most important: an open mind and a sense of humor.

The rest of my list is #14 here:
http://www.lonelyplanet.com/thorntree/thread.jspa?threadID=828622

The one absolutely essential thing that you need is your passport, which doesn't expire within six months. And, depending on where you're traveling from, any other travel documentation you may need.

Power adapter? Where are you coming from?

Very difficult to find binoculars in CR...if you want your own, bring them!