Where are you staying when you come to Destin?
Crab Island is fun but be prepared to get there early to be able to get in the middle. The closer you are in the middle equals less of a current (especially if you are taking kids) and closer to the water jump castles and trampolines for the kids. The farther out on the edges means more waves from all the boats and more water currents. I have seen many people, including kids, get swept away by the currents when not expecting it. If you have a pontoon, there are many great places along the beach you can beach it and go swimming and snorkeling also.
Harbor Walk is a lot of fun with a lot of places to eat (McGuire's Irish Pub, Jackacuda's, Crab Island Cantina, Harry T's, Margaritaville) and if you get there later in the day, you can see all the fishing charters coming back in with their daily catches. This is also where you can charter fishing boats or go out on dolphin cruises, pirate cruises or party cruises for the adults.
If you are willing to drive a little, there a many great places to see and do in between Destin and Panama City Beach. I would highly recommend going to The Village of Baytowne Wharf at Sandestin, Seaside Beach (The Truman Show was filmed there), Grayton Beach and Rosemary Beach.
You can also find a lot of things to do by just driving down scenic Highway 98 and Highway 30a.
Have fun!