Cypress Gardens is looking pretty good these days, new rides and an excellent deal on entry.
The Wilderness Preserve in Poinciana (20 miles south of WDW ) is a pleasant ( and cheap) afternoon trip. You can take a walk along one of the hiking trails there, they also do "swamp buggy" tours and a guide will explain to you how they are trying to preserve the Florida wilderness as well as some of the animals and birds they have on the preserve. We spent about 4 hours there I'd guess, if we'd taken the longer hike that could easily stretch out to 5 or 6. We saw deer, a coyote, a racoon, gators and plenty of birds. We saw tracks from foxes, a bobcat, wild piggies, gopher tortoises and armadillo. It's perfect possible you could run into any of these when out on the hike ( as well as a number of different snakes that live there

so do be aware of what lives there). The guide also told us that for the last few years a female Florida panther had been bringing her cubs into the preserve for a couple of months each year as they grew up. You could easily couple this visit with a trail ride at "horse world" also in Poinciana, they run trail rides through the woodlands/nature areas around Poinciana. I think the rides vary from 1-3 hours. Also in that area is Green Meadows petting zoo/farm, again it's probably not more than a morning/afternoon's entertainment, but if the kids like animals etc it's a workable option to dovetail with either the horseworld ride or the Wilderness preserve for a whole (busy) day's activities or all could be used as a half days entertainment to go with a lazy half day round the pool.
Crystal River would be another option for an overnight trip ( depending on time of year) , the manatee swims are best in January/Feb/march I believe.
I would also consider a couple of days at St Augustine, if you go to Hilton Head, it's an option on the way up/down if you don't go to HH it's an option instead and spend a couple of days there. Plenty of history there and some great beaches.