The best thing to do in Puerto Viejo, and really the only reason to come, is to relax. Chill on the beach, lay in a hammock, gaze at the stars -- if you're expecting a town with many activities, try the Pacific side.
If you are a bit bored, snorkeling trips are available, as well as moped, ATV, and bike rentals to explore the surrounding beaches.
Quick Tips:
Puerto Viejo is HOT, and the mosquitoes are plentiful, with rumors of malaria. Carry and use bug spray, and if you can, shell out the money for a room with AC, or at least a fan (being budget travellers, we did the latter, and sweated through the night).
The beach and waters are extremely rocky, and not welcoming to swimmers. Contrary to guide books, surfers may also be disappointed, as there are few waves to speak of, with the exception of "The Salsa Brava," which is outside of town.
In an attempt to be honest, I must say, especially for the younger set, pot is a big draw. If you're uncomfortable in such an environment, don't go out much at night -- everyone smokes, and I mean, everyone. This isn't a place for grandparents...
Best Way To Get Around:
San José has buses to Puerto Viejo every day, with the first leaving at 6am, though it's frequently full -- book ahead, even during wet season. We missed the early bus and opted to go to Limón and take a transfer to PV, which was terrible. Try to avoid going to Limón if possible; wait for the next Puerto Viejo direct bus.
In town, walking will get around the small grid-like town perfectly fine. If you want to explore other areas, consider a rental, especially a bike rental which are just a few dollars for the entire day. You'll see signs throughout the town.