You may want to consider buying a cancun flight outright. I don't know where you are coming from, but you can normally purchase the flights for 250-300 if you watch for sales. I know it's free, but it isn't a good use of miles from a $/mile standpoint.
Unless you have more than you can possibly use, I would save the miles and fly to Hawaii or save them for an emergency flight that may cost you an arm and a leg.
And xel-ha was hands down the worst thing I've ever done on vacation. It was crowded, nasty, cheesy, and our excursion had a planned time to leave, so we were stuck all day. The food there is quite possibly the worst food I've seen anywhere. They bill it as river tubing and rock jumping and snorkeling. It was so bad all we could do was laugh.
[This message has been edited by Red04 (edited 2/5/2013 9:14p).]