Hi, you're definitely right about all the typical stuff to do.
I am a big believer in traveling and seeking out the local stuff.
What I recommend in Costa Rica are a few things. I'm not really sure where you're going, so I'll give you some ideas. Some places will have these options others not so much, but if you ask around you'll find tons of treasures yourself.
Ok, here's my local finds.
Always eat in a local soda (restaurant/cafe). They are super easy to find, they are tiny, very local and serve the yummiest food. I wouldn't ask a hotel, ask a local for a good soda.
In some areas there are tilapia farms where you can catch your own and they fry it up for you. This is super local, you will not find this in any guide book. You can find this around Sarapiqui area, in the Central Valley area especially in the mountains of Heredia, and also in la Fortuna area.
In Rincon de la Vieja area, near Liberia, you can go to mud pools, very few tourists do this and it's great fun.
Oh, and don't overlook public, local transportation. They are so fun!
Hope this helps!
-snip-
[ Edit: Sorry, no promos please. ]