We did several things on Isla Mujeres and had a great time. The beaches on Isla Mujeres are a lot better than on the mainland side. There is a pretty significant seaweed problem on the Cancun beaches.

We went to Xplor and had a great time. The main attraction here is the ziplines and they are great. I was torn between Xplor and Xcaret. I think everyone who has seen Xcaret's dinner show has really loved it. Xcaret has more cultural elements and Xplor is more fun/adventure oriented.

We did a whale shark tour which was AMAZING. I loved it. We've been diving in the area before and I love that too. My only caution here is that if you were at all prone to motion sickness this might not be for you. We were 30 miles off the coast, the boat slows down and tries to get close to the whale sharks so you can jump off the boat and see them up close. While the boat is going slow and maneuvering it is rolling in every direction in the waves. Pretty much everyone on the boat got sick (Even with Dramamine).

We went sailing down there too. I enjoy sailing. The wind was good but the boat was really not a fast/performance boat (Which is more my speed).