If you really want to do the ruins on the Mainland, get the Adventure map mentioned by Don. And you will want to do it as an overnight trip, otherwise you are going to be traveling 6+ hours each way to get there. Sorry, but that is the problem with those southern hotels.
robin
Depends on what ruins you want to see, if you are going to Chichen Itza ,Ek Balaam or any number of sites located in the "heart" of the Yucatan, yes you have quite a trip. But if you want to go to Tulum or Coba, it isn't that far and it's a good way to kick off Mayan intrests and can be easily done in a day.
There are tour busses that go from Playa del Carmen to both sites, you can arrage them through most activites desks at hotels and around town, but I prefer to ferry over and rent my own car and drive down there myself. The tour busses often are just a 1/2 day at the sites and then a 1/2 day at Xel Ha or some theme park like that. You have much more time to explore if you drive yourself.
Tulum is at the most 2 hours south of Playa, but I think it's closer to 90 minutes.
Then to get to Coba, go just a couple miles south of Tulum and turn right, it is just about 40 km into the jungle from Tulum. I have driven both, more than once, but trying to make both sites in one day is a little hard unless you get a very early start. If you decide to try it, I'd go to Coba first and then enjoy the coolness of the ocean breezes at Tulum in the afternoon.
We drove to Coba and hired a guide once we were there for about $25, plus tip. It was great! I've been studying the Maya for several years and we had a great conversation along the way. There were several guides for hire standing just inside the gate when you paid, it was well worth it.
Tulum is much more excavated and a lot cooler due to being located on the coast line. The temples are quite small compared to those at Chichen Itza or Coba, but there is a lot of very cool history there.
Coba is not nearly as excavated as Tulum, Chichen Itza or even San Gervasio on Cozumel for that matter, but it does have the tallest temple on the Yucatan and an excellent example of a Puuc style Mayan temple...you have to go to the Marida (sp?) area to see another one of those.
There are 3 little Mayan villages on the way to Coba, in one of them, I think the second one, there is a guy who carves hieroglyphs (sp? again) out of limestone blocks and makes other Mayan statues and sculpturs. I have several of his pieces, they are VERY cool soveniers.
There is also the option of flying to Chichen Itza, I think it has become a little expensive, but that's the way we did it the first time we went there. You fly out of Coz to the C.I. "airport", more of a runway cut out of the jungle. Take taxis to the site, have a guided tour, I think lunch is included, it's pretty much a full day affair and Chichen Itza is worh every penny if you can swing it.
Getting back to San Gervasio, it is a Late Post Classic site, so much like Tulum the temples are small, but the site is much larger than I had anticipated when I was there, and again it is a very good place to kick off Mayan interests and can be easily done on a dive day afternoon.