I just got back from Aventura Spa Palace in Tulum, Mexico. Its really cheap and you can find lots of member selling their weeks and you get a reduced rate plus tons of incentives (like free pedicure, manicure, massage, hydrotherapy access, free transportation from airport, plus more). Plus they have included excursions to ruins and other fun stuff. They have a lagoon you can snorkel in out front and I thought the food was fabulous. You were treated like royalty at this place! If you sit down its mear moments before you are waited on (whether at the lagoon, on a bench somewhere, etc). There was never a moment where there wasn't a drink in hand. This is mostly all couples here. I was surprised how good the food was. Its about an hour south of Cancun so it gets you out of the hustle and bustle of the commercial setting.
My honeymoon pics (not all up yet):http://public.fotki.com/hurricanejedi/2006/honeymoon/
Palace Resorts Website (Aventura is the one we went to):http://www.palaceresorts.com/
Tripadvisor is the best place to research any resort. Click on "forums" at the top and it will take you to the Tulum message board where lots of ASP members will be willing to sell weeks and tell you about the resort. This is hwere I did all my research. My husbands coworker is a member and we went on one of their weeks.http://www.tripadvisor.com/Hotel_Rev...Peninsula.html
Edit: Palace Resorts also has Cozumel Palace. The nice thing about this location is they offer a free glass-bottom boat tour where you can go snorkeling. I've heard its really good but didn't have a chance to ferry over (you can switch between the palaces for free too).