I have done both the excursions to the beach, and across at Playa de Carmen. I really enjoyed Xcaret & Xel-Ha. I recommend both. With the Xel-Ha excursion, you can visit the Mayan ruins as well as enjoying the park at Xel-Ha. My avatar is a pic I took at Xcaret. It was beautiful as you can see, and there were plenty of things to do there as well.
The beach excursion I took on the island of Cozumel was fanastic. It had a protected beach, and chairs w/umbrellas. They transported us there & back. We had fantastic food, and lots of fun.
Good luck with your choice, but be sure to return in the future to do some other stuff! 
Happy Cruising! - Is there any other kind?

All work and no play makes FuzzyFern completely insane!
Cruise History:
Oct., '76 - NCL - Haiti; Dominion Republic; San Juan, PR; St. Thomas/St. John, USVI (7-day out of Miami, FL)
May, '91 - Carnival - Ocho Rios, Jamaica; Grand Cayman; Cozumel, MX (7-day out of Miami, FL)
May, '05 - Carnival Conquest - Montego Bay, Jamaica; Grand Cayman; Cozumel, MX (7-day out of New Orleans, LA)
Feb., '06 - Carnival Elation - Progresso, MX; Cozumel, MX (5-day out of Galvestion, TX)
May, '09 - Carnival Facination - Key West, FL; Bahamas (5-day out of Jacksonville,FL)
** Planned ** 2012 - Southern Carribbean island hopper cruise!