
If you drive a half hour or so south on route 307 you'll hit Playa del Carmen, which is very nice, full of open-air restaurants and cute little motels and some quantity of culture. South of that is Akumel, which is nicer still, particularly because of the snorkelling in Yalku Lagoon and the lovely beaches. Tulum is maybe 2 hours south of Cancun, on the map it is just south of Club de Golf Puerto Aventuras. Tulum is extremely excellent, mostly because you can rent a little caban˜a on the beach and eat awesome Yucatan-flavored seafood while watching the waves come and go. Cobá has some fantastic ruins, and is more full of jungle (we saw monkeys there).
At the end of December it was amazing sunscreen & shorts weather. All the beaches we hit were very swimmable. Here is the beach at Akumel:

I really liked this area, but I recommend against staying in Cancun itself. It's a big city of enormous, absurdly priced hotels. We stayed at the Holiday Inn there our final night, probably would've gone back to Tulum if we'd been less tired and hungry. We had to wear plastic bracelets so that the security would know we were okay to come and go. An hour later I wanted to gnaw my wrist off, Peter had to cut the bracelet off with a key. Eesh.
Akumel, Tulum and Cobá all get an enthusiastic thumbs up. More about that coming soon!