We did the Tulum ruins in 2005. After a ferry boat ride over to Playa del Carmen, you hop on a bus ride for about 45 minutes or so to the ruins. They do make one pitstop to use the restroom and shop for souvenirs.
The tour guides were great and very informative about the Mayan culture and area. The one disappointment we had was not being able to actually walk into and or climb the ruins. You can only view them. Even the tour guide said the Chitzen Itsu was much better if we ever came back to the area.
There was a nice small beach to relax and some Mayan culture shows performed at the main shopping area. We really enjoyed it, but it was just my Dad and I who visited. My DW and Mom stayed with the kids and I don't think kids younger than 10 would enjoy the visit to Tulum as it was very hot and there wasn't much for kids to do.
The provided lunch wasn't much, a warm (because it'd been outside) Subway sandwich and chips.
On the ferry boat ride back to the ship, the group that visited XCarcet talked about how fun it was there, especially the river. If you're wanting educational and enjoy history, I'd say visit Tulum, but if you're looking for fun, try XCarcet or another excursion.