Oh yeah, at least a few days in San Pedro. My first trip I split between mainland and AC. My second trip was all AC.

Snorkel Hol Chan Marine Reserve and Shark Ray Alley with Alfonso Graniel. He's extremely knowledgable and a total blast. Once you get to the island, local tour companies (at many hotels) can book you with him. You can do both in a half day.

Cave tubing was enjoyable. I liked learning about the leaf cutter ants along the trail to the river, seeing the bats in the dry caves and tasting the treasures the guides found for us along the way. Then, the cool water is a welcome, relaxing ride. We stopped at some gems of swimming holes along the route. It's a wonder to to see the jungle canopy through openings in the cave ceilings.

Be sure to do a Mayan ruin while in country. Lots of folks have fun with the zip line tours as well, although I haven't done hat one myself...yet : )