With that many people, I definitely suggest renting a van. There are several car rental companies within 3 miles of the pier. They'll pick you up and drop you off for free. It'll definitely be ALOT cheaper than purchasing any excursions, that's for sure!
The Kennedy Space Center is only about 20 miles away and is wonderful for kids of all ages.
Disney world is only 65 miles away, but you really don't have enough time for that considering you'll spend about two hours driving there and back. Same is true for Sea World.
Unfortunately, there's just alot to do in the immediate area, so you basically are limited to KSC or the beach - and there are some nice beaches in the area.