Another shameless plug for the TECO Manatee Viewing Center in Apollo Beach. I am a volunteer docent there and it is a wonderful place and 100% free. No charge for admission or parking. There are picnic tables there so you can bring your lunch to enjoy at the center. We also have a small education center/museum as well as a small snack bar and gift store. All profits from gift store and snack bar go back into improving the center.
Be sure to walk down the tidal walk as well as the viewing platform. We never know what we are going to see out there besides manatees. There are almost always some smaller sharks, tarpon, stingrays, and various gulf fish in the canal. Of course, wherever there are fish there will also be shore birds, pelicans, egrets, etc. You will probably see hoards of tiny fiddler crabs but I've also occasionally seen a horseshoe crab and blue crab.
We also have a very nice butterfly garden and butterfly exhibit with monarch caterpillars in various stages of chrysalis. I actually search the garden for the caterpillars, put them in the enclosure with milkweed, and once they hatch and the wings are dry release them back into the garden. If you are lucky, you may actually see a butterfly hatch.
One word of warning though, if it has been very very warm there may be very few manatees in the canal. They are only in there for heat when the bay is below about 68 degrees. Last Wednesday there were only two manatees that I saw in the canal.