If you don't want to go to a park:
A late resort character breakfast (expensive, but we've done the checkout day character breakfast so we don't spend time on a park day doing it).
Resort hopping (especially if your kids are a little older). Checking out all the resorts (you can't swim, you can check out lobbies, lounges, grounds and gift shops), can be a lot of fun and you start planning your next and your dream vacations. It could be a good "ice cream for lunch at Beaches and Cream" day.
Downtown Disney/Disney Springs.
Hanging out at the pool.
Our last trip involved a last day Typhoon Lagoon day - at 3:00 we changed in the changing rooms at Typhoon Lagoon, threw our swimsuits in ziplocs, and headed out.