One of Florida's most famous attractions is the Everglades National Park. The Everglades National Park is nearly 2400 square miles and is a fun place to explore. Be prepared though - it is a giant swamp which can mean a lot of mosquitos and hot humid weather.
Royal Palms Visitor Center is one of three visitor centers and is a good place to start. It has a helpful little museum with a movie. Kids can also get their Junior Ranger activities here.