Snorkeling
Snorkeling around las tintoreras or concha de perla is excellent. you might also ask a local panga owner to take you out to "los tuneles", or "the tunnels", a wonderful area of eroded lava islets filled with sea life.
Hiking
There's a nice 2,5-3 hours return trip hike to the wall of tears. on the way there are lots of things to see; lakes with flamingos and other wildlife, beaches with hundreds of really big iguanas, beaches with sea lions, giant turtoises and there are some view points. take plenty of water, sunscreen and a hat.
Visit the tortoise breeding center
A nice half hour walk from town through mangroves to the breeding center. some information and lots of giant tortoises free
Diving
There is only one diving center on the island which offers dives for 120-140$.