Go to dinner in a nice disney restaurant one night that is not in a park (Ohana, Boardwalk and DTD restaurant). Water Parks will be packed on your trip. If you want to go to TL or BB, get there somewhat early because there's a good chance you could be turned away (maybe go on a slow day of the week). Another idea for you is minigolf.
If you do decide to go to a water park, you may want to get a park hopper with water park admission. You could enjoy your evening in a park and watch the fireworks. Maybe someone can give the exact price difference in a water park ticket and the Park Hopper option.