I have been to Playa del Carmen and Yucatan some years ago and of course there are great things to do. In P.dC there is a wonderful white beach - so swimming is great. There is Xcaret which is a park where you can swim through an underground river and do some gorgeous snorkeling. You can go to Tulum and see the Maya temples (plus some wonderful swimming).
An excursion to Chichen Itza is also recommended (if you feel ok driving on the right side I would recommend renting a car.)

Most of the hotels are all-inclusive or at least half pension- which is a pity because we found that the restaurants in the village are much more fun, so we skipped our half-pension and went donw-town ( ahm village) every evening.

Of course California is great as well- but totally different. Yucatan is more exotic. At what time do you plan to go?