I recently returned from Cancun, Mexico. A short trip from LA, Cancun is excellent for 3-4 nights. Plan for lots of tanning, beach relaxing, and lux hotels. Hotel accomadations are plentiful, and plush. The beaches are beautiful, white sand, and tropical waters. However, if your looking for water sports, Cancun may not be the best option. Waters are somewhat rough and there is a consistent onshore breeze. If your feeling advantous and interested in partaking in some spring break fun.. downtown cancun can provide an interesting night. Tons of night spots, fun music, and a very, very young crowd. Or stick closer to the beach and bounce around the hotels for a more sophisticated feel and a surplus of dinning options. There is a rather decent mall in the city center offering most designer stores. I would definitely recommend checking out Thai restaurant located adjacent to the mall, for an interesting experience. Including; a water bar with live dolphins and some very intamate cabannas over looking the sea. The ambiance of Thai restaurant was extremely relaxing. For an authentic Mexican experience try Fred’s. Although somewhat Americanized, Fred’s offered a nice casual atmosphere and some excellent fruitful drinks. 
LA Donna 