Here is the final installment of my Yucatˇn suggestions.
Our final week in the Yucatˇn we stayed along the coast of Quintana Roo 3 miles north of Tulum on Tankah Bay. We rented a beach house for the week for our group of nine. You mentioned staying in a cabana which are mostly on the beach south of the ruins. I didn't make it down to that beach so I can't comment on it. However, we loved the area where we stayed. Far enough away from Tulum for peace and solitude; close enough for a quick drive into town for supplies or dining. The bay with its barrier reef and coral heads throughout offered superb diving with a multitude of species, including turtle, barracuda, manta ray, and a plethora of colorful reef fish. Also found here is a cenote adjacent to the beach which offers some spectacular and refreshing freshwater swimming and diving (in fact, you can dive through a passge from the cenote to the sea, with scuba gear).
There are two places where you could stay in Tankah, the Tankah Inn and Casa Cenote. Both are operated by Americans and offer both rooms and dining. You can get info about Casa Cenote at http://casacenote.com/ . Unfortunately, I have no contact information for the Tankah Inn.
Sights: The most obvious down here would be the ruins at Tulum. I've heard that these are the most visited ruins in the Yucatˇn. And it is easy to believe when you see the busloads of people arriving from Cancun. Go early! Although the setting is breathtaking, the ruins themselves do not compare with Uxmal, Chichen Itza, Ek Balam, or Coba. You'll need no more than two hours to visit them (and if its crowded you'll probably want out sooner).
The real draw down here are the beaches and the reefs. South of Tulum is the S'ian Kan Biosphere Reserve. Definitely worth a visit if you're interested in the wildlife and natural history of the area. We opted out on this trip deciding to return and focus on this area in depth at a later date.
We did take a day trip to the ruins at Coba and the spider monkey reserve at Punta Laguna. Coba is about an hour's drive from Tulum. An interesting site which is still closely defined by the jungle which embraces it. The site is huge and bicycles can be rented to get around. I highly recommend using them. It is actually quite pleasant riding along the wide flat jungle paths. And, as you said you'll be visiting in December, the weather should be mild (by jungle standards).
The reserve at Punta Laguna was a real treat. This is an area set aside for the preservation and research of spider monkeys. We hired a guide (the son of the gentleman who helped establish the reserve) who led us on an hour-and-a-half hike through the jungle. The curious monkeys gathered in the treetops above our heads including a couple of mothers with young babies. Also along the trail we saw army and leafcutter ants, a number of small Mayan ruins, a variety of birdlife, and were told by our guide about the different types of trees and plants and there uses.
A final note. All of these places I mentioned in the last three posts we managed to visit using public transportation (the exception being the day trip to Coba and Punta Laguna). So, if we (a group of 9 to 12) could do all this using buses, taxis, and colectivos you should have no problem getting around with the freedom of your own vehicle.
If there is anything else I can help you with please ask.
Buen viaje! John Rule San Diego, CA