We really enjoyed the ruins. The setting for Tulum is gorgeous, right on the coast, and very different from Altun Ha in Belize, which we also did. Some posters have suggested wearing your swimming suits and spending a little time on the beach right below Tulum. There were several people down at the beach, even though the day we were there was cool and cloudy.
We also had about 1 hour to shop in Playa del Carmen before taking the return ferry. I liked Playa del Carmen MUCH better than Cancun. We thought the shopping center (which was under construction) was a good place to shop -- but admittedly, we're not big shoppers. I would recommend going there at least once. We're going back again in May, and are trying to see more of Cozumel this trip, but I did enjoy the excursion.
The weather the day we were there was much windier than normal, which obviously contributed to the rough seas. In fact, we were on the NCL Sea, and we couldn't even pull away from the dock until about 2 hours late. The winds were blowing so strongly that they were afraid if they tried to leave the dock that winds might blow the ship back into the dock hard enough to do damage.
My sisters, who both get car sick, were with us, and neither of them had any problems. I was amazed that they didn't, but if you know you get motion sick, just take precautions.