DG, tubing is a gas, a great experience. The closest one is the Jaguar Paw trip that includes lunch & the belize zoo as well. I have used Tanisha Tours for this one, I did this tour several years ago. Last year I used them to do the Lamanai ruin tour which was just wonderful. Gives you a great overview of the interior as you travel up several rivers by boat as well as a bus jaunt from bomba village in between. It's a full day with lunch included. Very well worth it.
On the island, be sure and get off the tourist track, get into the neighborhoods, eat from the street vendors and rent a golf cart after you get there and tour the whole island. It extends well to the south of San Pedro town.
You can use the water taxi's to go to caulker too, it's not far, but a different vibe I feel. I love caulker. You can rent a bike on either island. You're gonna have a great time, I've been going routinely for 5 years now & it has become my winter haunt. Hope this helped.
