If you're staying in Puerto Viejo, I enthusiastically recommend taking a day trip (or stay overnight if you're really adventurous) to Yorkin, a village of indigenous Bribri Indians. You start from Bambu (about 45 min. from Puerto Viejo) and take a dugout canoe up the Yorkin River. You then hike up to the village (about 20 min.). The Bribri are extremely welcoming and warm. They show you how to make chocolate from the cocoa beans grown nearby. You get to taste it, of course, and it's like no other chocolate you've ever had. They make a great lunch for you, too (the monkey tails--a fern-like green--were awesome). They also took us to swim in a nearby river that borders Panama. (Wearing river shoes helps ensure you can make it across the current back to the Costa Rican shore.)
We booked our trip to Yorkin through ACTUAR, a community-based tourism group in Costa Rica. It really is a remarkable trip. And knowing that you're supporting an indigenous culture makes it all the better.