Fitbit Ultra
If you're a Fibit user, don't forget to bring your Fitbit! You'll probably be doing lots of walking, racking up steps like crazy. Fitbit is a great way to keep track of distances, especially on those winding rainforest trails.
There's a Fitbit user's group in Costa Rica on the Fitbit website
Don't forget your base station. But if you did, ask on the group, someone should be able to meet up to sync your data.
Remember to set your tracker to record every time you get into a car or bus. The roads in Costa Rica, especially rural roads, are full of bumps and will generate tons of false steps you'll need to prune later!