Cancun is nice, but I prefer to stay in the Riviera Maya area about 40 miles away. Less hustle and bustle. The resorts in the Riviera Maya are are great and price is right. Plus there are ruins nearby, good snorkeling, dolphin swim, etc.
Puerto Vallarta is nice as well, but I book more to the Riviera Maya area because from the east coast it is closer. Resorts like Westin, Club Hotel Riu, Camino Real all beautiful hotels.
Also with some of the prices for Caribbean going up, you can do Hawaii for about the same rates.