You don't need an excursion (lot of them I have seen tend to be booze cruises), just take the Ultramar ferry over, either from Playa Tortuga in the Hotel Zone or from Puerto Juarez (further from your hotel but more frequent service).
Snorkeling is decent at Garrafon de Castillo and near the bridge at Mia Reef close to the end of Playa Norte. Plenty of places to eat, maybe try one of the seafood places along Medina (street where the ferry docks) with toes in the sand.
I totally agree with 55. Especially since you're going with children, it's much nicer to go and explore the island at your own pace. Plus it's super easy to take the ferry.
I would recommend going early and making a day of it. You could rent a golf cart and head down to Garaffon to snorkel, make a stop for lunch, then head back to the north end of the island to swim on one of the beaches before heading back to the mainland.
I agree with previous poster. The tour we took was a boozy event and I wouldn’t have wanted my kids there. We had a great time but I would recommend it for kids.
where do you rent a golf cart? Is that something we can access easily from the ferry dock when we get there?
Lots of places to rent from straight across the road from the ferry dock,you can get slightly larger ones with forward facing rear seats for 3 kids.You will need your drivers license.
If you are staying at the Westin Lagunamar the closes ferry over to Isla Mujeres is near Coral beach but there are more frequent departures from Playa Tortugas. Puerto Juarez is a bit cheaper but further from the hotel district. Once you are at Isla Mujeres there are a lot of places to rent golf carts. 2 hours so be good and from the same dock you arrive at you can ask rates for snorkeling. As for excursions you can find family themed catamarans, double decker boats that also have family atmosphere and boat that take you to swim with dolphins at Isla Mujeres. Isla Mujeres is not very big but fun to explore.
As others have noted, take the ferry over to Isla from the Hotel Zone, I'd suggest possibly picking up a map if your first timers from MapChick http://www.mapchick.com/isla-mujeres-map/ prior to the visit.
Rent a golf cart, there will be plenty available if you arrive early, most folks will spend from $40-$55 US for the day depending on season and type of cart.
Once fully mobilized or on foot for that matter from the ferry dock, go left on Medina Avenue, pass through the road at the Privilege Aluxes Hotel and you'll find yourself at Playa Norte, a beautiful expanse of beach with many beach clubs. Along the way you'll pass Jax on the corner, another popular casual sports bar type spot and the Privilege Beach Club if interested.
Looking for shopping, fun and reasonable foods, take a walk down Hildago Avenue near Playa Norte and enjoy.
Or, hop in the golf cart and drive the island perimeter, there' various snorkeling and dive shops with options, or find the South end beach club just before the Eco Park El Garrafon and check out the beach snorkeling options and day costs. ;-)
Cost - read post #7.
Tickets can be purchased in pesos or US$
Yes you can just walk in and purchase tickets at Playa Caracol,they will ask if you want to purchase tickets for the golf cart on the ferry but it cheaper when you get there. When you return because the ferry times are fairly limited you can change your ticket to Turtugas and then catch the bus back.