OK first thing, you are planning on going during the snowbird season, at least for Feb-Apr so expect elevated prices in the CG and difficulty in finding a site. At least below Orlando and near the coast lines.
Florida has many fabulous springs, most are now the centerpiece of a State Park. I'd recommend Manatee Springs State Park (may have manatees), Blue Springs State Park (manatees), Rainbow Springs State Park which also has the crystal clear Rainbow River.
There are tour operators which provide snorkeling and Scuba trips of the Rainbow River. You jump in at the head of the river and float downstream with the current. The boat follows along.
For canoe/kayak, tube trips check out Juniper Springs (in the National Forest) and Ichnetucknee Springs State Park. Also the Peace River near Arcadia Fl. The local Canoe Outpost organizes trips where they drop you off upstream and pick you up downstream. The Peace River is a good place to find fossil sharks teeth and other fossils during the canoe trip. In fact there are Canoe Outposts all over the state the do the same:
LinkOnce you are north of Orlando and in the middle of the state you should be able to easily find campgrounds with open sites.
The state has over 90 state parks now, so many that I cannot keep up with the new ones. Even if you can't camp in one they are still good for a visit.
State ParksHomasassa SP is a guaranteed place to see manatees since they have a captive herd of manatees that are injured and can't be released to the wild. They also have a lot of other critters since the park was a privately owner attraction that was taken over by the state. Silver Springs SP and Weeki Watchi SP are two more attractions bought by the state. They are in the same general area.
There are also many state parks on the coast lines for swimming. Some have camping and some don't but all will be difficult to get a site in.
Ybor City and Tarpon Springs are two towns that somewhat retain their old character. And St Augustine is another.
Near Sarasota is Myakka River SP, you will see more alligators than you ever dreamed existed in that park. They also have a sky walk which is a walkway up inside the trees. Lots of birds. If you like birds then Ding Darling NWR is just south of Sarasota:
NWRStrangely enough the St Augustine Alligator Farm is a fantastic place to get withing touching distance of nesting water birds (egrets herons ect)
And speaking of gators, the Everglades NP Shark Valley location off of US 41 has many fearless gators and birds. They have a tram tour which will take you around the loop trail or you can rent a bike.
Sarasota also has the Asolo State Theater which was a theater from Italy that was taken to pieces and rebuilt in Sarasota. And the Ringling Brothers Circus Museums.