If you enjoy history, then the Tulum ruins are a must!!! They are excellent. The ruins on Cozumel can't compare to these.
We combined this with Xel-ha for some snorkeling and watching people interact with the dolphins (not enough time for us to do this, but still alot of fun to watch and take pictures of) and it made a fantastic day. It is a full day and you won't have much time to explore Cozumel, but I've been to Cozumel before and it doesn't compare to Tulum.
If you don't want to do Tulum and you're going in June or later, you may want to check out the state park (Ch..K.....). It was severely damage by Wilma, but is expected to be better than ever when it re-opens. I snorkeled there on a cruise in 1994 and it inspired me to get my diving certification. The snorkeling is wonderful and there are many colorful fish!!!
Have a great time!