Not sure there is enough to do just land based.
We Scuba dive and did a trip to Cozumel, flew into Cancun, took bus to Playa del Carmen for a few days doing land base stuff before taking the ferry to Cozumel for a week of diving.
We liked Playa del Carmen lot of stuff to do. Shopping, eating, tours, ruins, Snorkeling
Here part of a trip report I did for the dive board
Other Information:
We like Southwest Airlines, they have a Non-stop from Denver to Cancun. We are retired and have time so we thought we'd do a few days in Playa del Carmen. We did the Bus to Playa del Carmenand, the drag to our hotel (was easy). We found a taxi that gave a 200 peso hour rate. Had him give us a tour of Playa del Carmen and two days later had him take us to Aktun Chen ( I like to have someone else do the driving so I can look around). We liked Playa del Carmen, it was a fun place. We did a trip to Aktun Chen to snorkel in a Cenotes (rated the best for snorkel). What I did not know is they have a walking tour of 3 Cenotes that were more like small caverns. Well lit and interesting formations. It was great. Think we liked it as well or better than the snorkel cave, which is good. If you think about it the Cenotes are formed with water in the bottom half so there is nothing to see in the bottom half , you have stalactites but no stalagmites. I would say do the cave tour if you can only do one, we did both. Other have given excellent advice on the bus and ferry so not going to add any thing here.

http://www.scubaboard.com/community/forums/cozumel.208/ ( lot of information not just dive stuff)

All in all, it was a great trip. We enjoyed the ruins at Tulum, Coba and Chichen Itza. (Note from the wife – “Not to be missed” is the evening dance show at Xcharet Park. At 4:00 the entry fee drops a bit, and since we had no particular interest in their beach or inner-tube river rides, it left us with time to see the grounds, the aquarium and butterfly pavilion before the show. We were both awed with this evening performance that gave us everything from history, to the Mayan ball games, to amazing musical performances. Even my husband was impressed with the stage engineering that was involved. Great fun!)