Ive been down there a couple of times and LOVE it.
I think you have to do Chichen Itza. It will be an all day trip for you, but I think you have to go there once. It is a bummer that you cant climb the pyramids anymore, but I think its still worth the trip. If you dont want to do an all day thing to see ruins, Id go check out Tulum. Its pretty close to where you are and has some awesome views of ruins by the beach.
My favorite place down there is Xel-ha. Ive gone there each time Ive visited Cancun. Great views of a beautiful place, some snorkeling, nice trails to walk, lazy floats down a jungle river, and a little cliff diving. The snorkeling isnt the greatest, but its still fun. Ive seen groupers, tuna, sting rays, and all sorts of other little fish there. Though another time I went I didnt see much at all.
Puerto Morelos snorkeling is another great place to go if you dont scuba and really want to see some nice reefs. Ive been there a couple of times and was happy with both trips.
A couple things I was really disappointed with that Id stay away from. . . there are a couple of zip line places that are retarded. In Cancun there is a tour that lets you drive a two man boat into the "jungle" and then out to snorkel. The jungle lasts about 10 minutes and then you just head out to sea to a lame snorkeling location. Not worth it. There is also a Wet n Wild there that is terrible. Ive also done a dolphin swim and was pretty disappointed.
Hope you have a great time. Definitely check out Xel-ha!