First off, I'm sorry.
But, as long as you're stuck there, there are fun things to do. Definitely take the kids to Hidden Worlds (http://www.rainforestadventure.com/hidd ... ds_mexico/
) or Xcaret (http://www.xcaret.com
) natural theme parks. They're expensive and a bit Disneyed out, but much cooler than your average Orlando nightmare.
If you're on some kind of a package deal, your hotel probably includes day trips to Tulum and/or Chichen Itza, which you should definitely take. If not, Tulum is an easy day trip by bus or shuttle from Playa Carmen, you don't need to book a tour. The ruins are mediocre, but the beach element makes it nice. Chichen Itza is probably cheaper and easier on a guided tour. It's overrated and crowded, though - if you don't get a deal on the tour, think about going to Coba instead - it's much cooler and less crowded, and it might actually be a little bit closer to you. It's a huge, only partially excavated city that was ten times the size of Chichen Itza, and you can rent bikes and go through dirt trails through the jungle and pyramids and whatnot.
If your family likes to scuba dive or even just snorkel, well, there are probably all kinds of day trips to Cozumel and I've never covered Cozumel. But close to Cancun, there's an Island, Isla Mujeres, that has the world's largest underwater art exhibit, designed to be a reef where you can dive - I bet there are day trips with boats leaving right from Playa del Carmen: http://www.underwatersculpture.com/page ... mexico.htm
. It's pretty bad ass, and Isla Mujeres is nice, too - sort of the hippy version of the Riviera Maya:
Other than that, all-you-can-guzzle watered-down well drinks and skin cancer are pretty much the order of the day. Enjoy!