If the weather is still nice, something outside? Golf range, batting cages (some are indoors), go carts place. We have a Dave and Buster's which is like a grown up Chuck E. Cheese, that the kids eat and play games at and get tickets for prizes. Lunch/movies out? Indoor pool? A lot of gyms offer a one day pass for a reasonable price and have basketball courts, climbing walls, etc. Do you have any indoor water parks by you such as Great Wolf Lodge or Coco Key? Paint pottery for holiday gifts coming up? Shopping? Renting a bunch of movies and have a hang out day with pizza and a sundae bar for dessert?
And like a pp mentioned, sightseeing something in your area that you have not done before? Any National Parks by you?
These are some of the things we do when we have a day off with kids. They are scouts too, so we use those days to catch up on working on badges, etc. too. (and sometimes schoolwork!)
Have fun!