I learn more from negative reviews than glowing ones, so I'm going to focus on some things you need to be aware of, not all of them positive.
We 4 adults took the Tulum excursion offered by Royal Caribbean 3/6/14 (CZ02). It was well worth the time and the ruins were most impressive, though not as large as Chichen Itza, of course. Overall, the trip was good and I recommend it. I would urge you to take the ship excursion because it's an all-day trip, and, despite any real setbacks, we were the last people back on shipboard. Fair warning!
It’s a long excursion. We were among the first tours off the ship, and were taken to a dock where we took a ferry, about the size of a large tender, to the Mexican mainland, about a 45-minute ride. Much has been written here about seasickness, but our ride both ways was smooth and I did not see anyone green around the gills. It was actually a pleasant ride. Once on the mainland in Playa del Carmen, we found the tour starting point and met our guide. We set off for the bus area, which is a few blocks winding around some confusing zigs and zags through town. Unfortunately, our guide, who looked like any other local, had no placard or even an umbrella to hold up so we could follow him amid the crowds on the streets. Fortunately we followed the right people and ended up at the correct bus. The 40-minute bus ride to Tulum, is down a mostly 4-lane highway while the guide, who was of Mayan descent, told us a few facts about the Mayans that you could get in the first page of a Wikipedia article. He spent about half of the time pitching a "custom made" Mayan calendar made for any date you pleased, to be delivered at the end of the tour. I did not see any takers, which was encouraging.
We were dumped off for 20 minutes in a warehouse-like crafts gift shop with the usual Mexican souvenirs in great profusion. There were restrooms, which was good since the one in the motor coach was out of order. ("Motor coach"= bus with toilet, so I have to call it a bus.) Pulling into the parking lot, I was disheartened to see about 20 tour buses, but as it turned out, the Tulum grounds did not seem crowded. Points for that. Once off the bus, we walked through some shops and walked roughly a mile down a paved road to the ruins. There is a tractor-pulled trolley [$2US] that you might consider if you don't want to walk it, but you may find it difficult to re-join your group. Regathered at the "gates" of the Tulum ruins, our group walked through a low doorway, and spread out in front of us was a large open area with quite a good number of buildings. It is a magnificent site, with far fewer trees than the Chacchoben ruins we visited near Costa Maya. The guide gave us some background (again, not very informative), and took us around as a group to the high points of the ancient complex. He then let us tour on our own and told us to meet in a half hour, for about an hour total at the site. We enjoyed the free time, but people with mobility problems should know that there are many steps and some small hills to navigate. Still, you can stay on the flat areas and enjoy it.
The adjacent beach, not included on our tour, was spectacular and very tempting--it was early March, but it was very hot and humid. Dress accordingly. We were in a rush to get back to the bus and caught the trolley going back because it was a bit faster. Rushing through another tourist trap souvenir area, we piled back on the bus and were given a box lunch consisting of a kiddie-lunchbox-style tuna and crackers, and water. Better than nothing. After the bus ride back to Playa del Carmen, we threaded our way through the streets (again, trying to follow a nearly invisible guide) to--ready for it?--another gift shop. Seems like no matter how rushed or late a tour is, there is always time for another gift shop.
Then we were hustled off to the dock and a guy handed us our ferry tickets and standing immediately next to him was the person who took the tickets--true south-of-the-border efficiency. The ferry ride back to Cozumel was uneventful, and back on land, we were greeted by the RC excursion director who was waiting for us, saying that we were the last ones to board the ship. Again, fair warning…take a cruise-sponsored tour! We took apples and bananas from the Windjammer buffet with us, but these were confiscated by Mexican authorities at the dock on our return. We didn't need them anyway, as it turned out.
The RC website description of the excursion mentions a $7 taxi ride, but we did not need this, or were even aware of it.
So even with all these issues, I would still recommend the excursion, but I hope you get a better guide.