if you stay in the Xceret area, I'd recommend not only the Occidental Grand Flamingo but also the following:
The Xceret waterpark next door -- you can do this thing where you float down the river that runs the length of the park. It's way cool
ATV tour -- it's about 2-3 miles from the hotel. You want that one. You ATV, then swim in a Cenote, then ATV more, then visit a Myan cave/alter, then ATV back
Dolphin Discovery -- This is the tour you want. Foot-pushs *and* dorsal fin tows
From the Grand Flamingo, they can take you to their sister hotel that has a *ton* of jetski rental and parasailing
Captain Hook -- you have to drive or taxi into Cancun, but it's a dinner/party boat where you can drink & dance. It's a cute show.
The Plunkmeister knows how to do Cancun!