O'Leno State Park is another wonderful place in florida to experience a bit of the 'real' florida. The park offers camping, play ground, fishing, sink holes. I am not sure if swimming is available at the park. I have never seen anyone swim there, and I would prefer not to swim in the rivers in florida.
I instead go for the nature trails, and to experience the wildlife, such as racoons, alligators, snakes, and the unique plants and natural habitats. I enjoy crossing the beautiful Suspension Bridge that stretches across the entire length of the river. It is a bit scarey to walk across this bridge as it feels as if it will come undone at any momment, but I assure you, I have been across this bridge several dozen times.
Cabin rentals are available, I would call months in advance for a cabin. Be sure to bring your own sheets, towels, etc. While the cabins are a treat, you are actually roughing it a bit. Try to get reservations during the cooler months, as there is no air conditioning in these cabins!!
Bring bug repellant. The mosquitos are so heavy in this area, you will have a terrible time if you do not.