Isla Mujeres is a small island just off the Mexican coast, north of Cancun. There are ferries to the island from several locations in Cancun with the most common being the ferry terminal just north of town. It's about a 15 - 20 minute ride out to the island via hydrofoil and they run all day and most of the night as well.
The town occupies the north end of the island where the ferry terminals are. The rest of the island is less sparsely populated and also has a small airstrip and a combination theme-park / national park of the sort that can only exist in Mexico. We didn't go there. The island can be covered on a rental bicycle or by taxi since very few destinations are outside of the town center anyway.
Unlike any other portion of Cancun, Isla Mujeres has a definite laid back Caribbean sort of vibe to it. There is very little traffic and no high rise hotels or giant night clubs.
One attraction of sorts on the island is the turtle farm which is primarily for rescued or injured turtles but has a pretty large turtle nursery as well.
Along with several large outdoor tanks there are also indoor tanks for turtle hatchlings. There is also a small aquarium of sorts with a few other local fishes and of course, seahorses. If you try fairly hard you can probably spend an hour at the turtle farm but it's a cheap taxi ride from town and worth the trip. Especially if you're entertaining two kids.
The main attraction of Isla Mujeres might be its beaches, particularly Playa Norte which is just outside of the center of town. This is by far the nicest beach we visited in the Cancun area. This is at least partially because it still has sand (Hurricane Wilma spared it) but more so because the water is so calm and so shallow.
The water here is about 3 feet (1 meter) deep as far out as the turquoise color can be seen. It's a great place to sit in a few feet of water and enjoy a tropical drink.