Go see the ruins at Tulum. It's gotten to be more of a tourist trap than it used to be, but still very interesting. Located right on the beach. Well worth seeing.
I also recommend Xel Ha (pronounced Shell Ha). You can go snorkeling there and lots of interesting stuff to see there.
I did not go to Xcaret, but it supposed to be a good place (but very crowded and expensive from what I've heard)
If you are into ruins, you can take a day trip to Chitzen Itza which is another Mayan ruin. They have a fairly large pyramid there. Also a "sports field" which no one is exactly sure what type of game was played, BUT it was a deadly serious game. The losers were put to death according to historians.
Lots of scuba diving/snorkeling opportunities on the coast or over near Cozumel.
It's a great place to go to, particularly this time of year if you are from Canada!!!
Enjoy, and watch out for the taxi drivers. Those guys are out of control!!