We're leaving in a few days and have a day trip planned to the Gulf Cost side to take a 4 hour Dolphin Cruise that also spends a few hours on one of the keys to allow for beach and water play time. It's really inexpensive at $30 per adult and $20 per child 12-4.
Here is the link so you can check them out:
http://www.tropical-island-getaway.com/