Swim into Blue Cave
With nothing but a headlamp and tour guide. find yourself sitting underneath an underground water fall deep into a cave system with the lights out. gain a new respect for flashlights.
Watch the sun set
From el pescador and stop by hickatee's for some great meals. conch chow mien highly recommend when available.
Walk the "Mayan Basket Gauntlet"
At lubaantun to see some great isolated ruins.