Our family went to Venado Cave several years ago. It is well worth visiting, markedly different from US show caves, to say the least. Take your own light source- the flashlights provided were fairly dim. Involves stream wading and some scrambling. Our boys were about 8 at the time, so we didn't look into exploring more of the cave, but I'll bet you could find a guide who'd take you throughout the cave for the right price. If you have a rental car, don't book a tour, just drive out to the cave and get a guide, you'll save a lot of money.
I tried to contact the Costa Rican caving group Anthros before we went, but had no luck. Perhaps you will. Check their web site- it's quite well done.
http://anthros.org/english/touristcaves.htmlA Costa Rican caver posted in the "Introductions" forum a couple of months ago. You might search that forum and contact her. Looks like there are some caves down near the Osa Penninsula if you are headed in that direction.
Mark