OK, you got me thinking.
Canoing on the Blackwater River, an easy trip with lots of places to stop and have a picnic. There are several places to rent canoes, they drive upstream and you float back to their base. Just the drive to the put-in spot is worth the trip, you will see all kinds of wildlife, wild turkeys, deer ...
http://www.blackwatercanoe.com/
Florida Caverns State Park near Marianna. It's nice and cool, a great escape from the Summer heat.
http://www.floridastateparks.org/floridacaverns/
Take them to a marina where they can see the fishing boats come in.
Big Kahuna?s Water Park in Destin. www.bigkahunas.com
We took our kids on a winery tour up near De Funiak Springs. The tour was fun but the wine wasn't so good.
Your kids might be a little young, but on a lonely back road they can sit in your lap and drive the car. It's probably going to cause a lot of folks on this forum heartburn, but it's one of the things I remember being a lot of fun. I even remember the car I first was able to drive, a 1953 Hudson Jet.
http://static.howstuffworks.com/gif/1953-1954-hudson-jet-3.jpg
Get some bait from a bait store and feed the fish, there's usually some large ones hanging out around the docks waiting for the scraps from fishing charters. Watch out for seagulls.