Beth:
If it's your first time to the area, then definitely do a Tulum/Xel-Ha tour. Your resort (or charter rep) will most probably offer one - usually a 1/2 day at each site, i.e. Tulum in the AM an Xel-Ha in the PM (or visa versa). It's the best way to do it if you're not familiar with the area.
I LOVE Cozumel. If you enjoy snorkeling, it's awesome there. Take the ferry over from downtown PDC and grab a cab to either Chakanaaab Park (which is a national park with lots to see) or Dzul-Ha Beach. We're going to Dzul-Ha this year for the first time (our trip is May 22-29), as I've been to Chak a few times and want to check out new digs, so to speak. Then a nice finish to the day is to shop a bit at downton San Miguel, catch a MArgarita or two and a bite to eat, and then hop the ferry back to the mainland. Check out Cozumel Insider to scope out what the cruise ship schedule is for that week
http://www.cozumelinsider.com/cruiseships/ You definitely want to try to do your trip out to Coz on a day when there are the fewest number of cruise ships in port. And also catch an early ferry to make the most of your day.
Send me a PM if you have more questions. I'm not an expert, but I'll try to help!! We're going back to Iberostar Quetzel this year for our 2nd time..... were there last June and loved it!
Oh... and don't fret about the heat in July. It's gonna be hot and humid.... so be prepared with lots of high SPF sunscreen, cool clothes, hats, and an attitude that sweaty is sexy!!
A cool attitude always helps!!!
Karen