An Ice Cream Social would be a really fun idea on a hot day!
These are just a couple of ideas from the countless things you can do to make their last day special. You could even incorporate something the kids might have learned about during the year like The Civil War, Native Americans, etc. You could do more traditional things like a luau, or an old fashioned picnic or even try to organize a mystery detective party or a scavenger hunt. The ideas are endless!