Camoed Bison Tube. Plenty of parking, short walk to the cache, nearby picnic area. Open daylight hours only.
Manatee Cove Park on Merritt Island, Florida is a great place to see the manatees. I've been to the park several times and have seen 5-10 manatees every time (unlike Manatee Sanctuary Park in Cape Canaveral that is located on the Banana River, and where I have never seen a manatee.) The manatees approach the shore and seem like they want to entertain people. Kayak rentals may be available nearby or you can bring your own (not needed to reach the cache). Non-motorized boats are also allowed. Great spot to bring the kids, easy trail goes half-way around the cove and offers several spots to observe the manatees.