I've done both, the bus tour and renting a jeep and drivig down myself. I prefer the driving myself bit. You get much more time on site and if you have extra time you can do some more exploring. Try and get down there early as the archeological sites close at 4:00p.
If you go in the a.m. and have some extra time, drive just a little bit south of Tulum and head twards Coba. there are 3 small Mayan villages along the road, in the second one, I think it is, on the right hand side of the road is a Mayan hut, out front you'll see a large (6'+-) stellae, a sort of carved monument.
This is a shop that an old Mayan gentleman has where he carves heiroglyphics out of 12" x 12" pieces of limestone, he also has smaller ones and some small statues and other mayan artifacts he makes, a very cool souvenir, better than anything you can find at the usual tourist shops. I've bought several pieces from him in the past.
If you want more mayan adventures, on another trip take the time to drive all the way to Coba, it's about 40KM into the jungle and hire a guide to take you around. you'll have to do it on a different trip, you can't do both cities in one day.
have a great trip, hope you enjoy Tulum.