I did the Tulum excursion & it does indeed take the whole day. I think we had about an hour leftover after the tour bus brought us back to the dock area, so we got to browse a bit - but I really loved the excursion & would suggest it to anyone who has never seen these kinds of ruins, or have an interest in history. If I recollect (it was October of 2009), we took a speedboat to another town (playa del carmen?) & then we took a very nice coach bus to the ruins w/ 1 stop for shopping at an "approved" local marketplace, then on the trip back we had bottled water & snacks. The tour guide can make or break any expedition, and ours was fantastic - you could tell he had an absolute passion for his subject & made the whole trip just top notch. One of the neat things you can do at Tulum that you can't do at Chichen Itza is jump in the ocean after you have seen enough of the ruins - there is a walkway down to a small beach area that can get quite crowded w/ tourists, but was a real treat after the heat of the afternoon. If you have problems w/ stairs - you won't want to go down - otherwise - bring a towel w/ you & wear a suit under your cloths! There were also plenty of small shopfronts outside of the ruins itself for you to shop some more & even get a bite to eat (chips & salsa & a nice cervesa!).
For what it's worth - I really enjoyed it