The little ones will love watching the boats go back and forth at Gran Puerto Pier. For a fun family day out, why not buy some tickets and head somewhere interesting?
You'll have no end of sightseeing choices when in Cancun. Gran Puerto Pier is an ideal place to begin here, but there are lots of other worthy attractions to experience. La Isla Shopping Mall, Playa Mujeres Golf Club, and Tortuga Beach are not far from the ferry passenger terminal and will round out your day nicely.
