There is the Wilderness Park on Fletcher about 1/2 mile east of I-75. (quality singletrack)
Flatwoods about a mile north of the Bruce B. Downs exit off I-75 which has like 30 miles of trails but it a hassle to ride to once you've entered the park because you have to endure like 4 miles of road bike track to get to the actual trails.
By far the best is Alafia State Park though... it has everything you would ever want to ride. Its off the Apollo Beach exit. Its DEFINETLY the best around here.