Isla Mujeres, or "Island of Women": I never really got a good explanation of the basis of the name, but it's a neat island to explore! I have been to the island three times now. Now, you can take a tour or excursion, or you can do what my wife and I prefer to do, which was to get there on our own. You can take a bus or taxi to Puerto Juarez. From there, you take a ferry that costs about $7 RT, and you take it to Isla Mujeres. Once I got there, I haggled my way in to rent a golf cart for the day. Normally about $50, we got it for about $32.
For fun, we like cruising around this little island, seeing some of the villas and homes scattered throughout and the nice views of the beach and ocean. We make our usual stop to the Garafon and go snorkeling there. There you will find a nice facility, as well as nice views of the sea. They have chairs, hammocks, and equipment for rent, although now we prefer to bring our own snorkeling gear. You can also swim in the pool that overlooks the water.
There are other things to do on this island, like seeing the turtle farm, swimming with the dolphins, and other things. It is simpler, yet we did notice more hotels popping up here and there. It is less commercialized than Cancun and definitely worth seeing!