I visited them all (and many many more) on a 2 week vacation last fall. 1st week was just driving around, visiting different ruins (16) on the peninsula. 2nd week was diving out of PDC (Playa del Carmen). You can't really go wrong with any of these places, except maybe Xel-Ha. (Xel-Ha is an eco-park, and I think Xcaret is much better.) There's pretty good descriptions of Chichen Itza, Tulum and Coba on this site:
http://www.mayasites.com/sitelist1.html
My impressions-
Chichen Itza: Wonderfully restored ruin, that has been cleared of jungle, allowing perspective. A whole day here can be fun. But it is about 2 hours from Cancun, and a little more from PDC. The road from PDC is nice, but not an easy straight shot, like the one from Cancun. Many bus tours run to this site, which can be nice to sleep on your way to/from, but that means it gets crowded, especially after 10am. If you drive on your own, I would suggest visiting the Dzitnup Cenote as well (not the cenote in Valladolid, but the one outside of town).
Coba: An enormous site. I would suggest renting bicycles once you get inside to see all of it. Much of the jungle hasn't been cleared, so it can be difficult to get a feel for the site as a whole. It was hard driving here, as I passed so many tempting divable cenotes on the way.
Tulum: Beautiful, on the beach, with little shade. Compared to the others, it is very small, but also very picturesque. Most of the pictures of ruins near the water come from this site. This can also get very crowded, as it's an easy day trip from just about everywhere tourists vacation (PDC, Cancun, Akumal).
My suggestion: take the ferry to PDC and go to Tulum in the morning. After Tulum, come back to PDC and have a nice lazy afternoon shopping and visiting the restaurants/bars. PDC is an awesome town, and there's TONS of stuff to do there. This will give you a good taste for the area, so you can determine how soon you want to go back!
I've seen all of these places very recently, so feel free to PM me with any questions.
Mike