Valley Mill has a camp that week and it’s usually still open this late in the registration season.
It’s not cheap but they have a bus from a number of DC locations that’s included and it’s a great way to wrap up summer
https://valleymill.com/. It’s VERY rustic facilities though, so if your child is easily grossed out by bugs, dirt, etc might not be for you. They get lots of swimming time and learn to do archery, build a fire safely, etc. The bus ride itself can be boring/chaotic but otherwise my kid comes home filthy and happy.