Ambergris Caye - Under the water:
Shark Ray Alley
Ambergris Caye /On the island:
Marco Gonzalez ruins
On the mainland
Lamanai - a day tour that leaves SP by boat and takes you to the ruins via a jungle river that is alive with amazing plants and animals - you will see rainforest, monkeys, tropical birds, orchids and one of the most naturally spectacular ruins in the area.
Easy day-trip - take the water taxi over to Caye Caulker, wander around, have lunch and come back mid-afternoon.
For the kids - if you need more to do - cave-tubing/zoo