At JFK there is a desk where you can store a bag in a secured room relatively inexpensively. IIRC I paid $20 to store 2 small bags overnight (they have different pricing depending on the size of your bag) back in 2018. I'm not sure if DCA has something similar.

You can also leave your bag at the bell desk of a hotel. I've never had them ask or care if I was actually staying at the hotel. If you tip then they will be happy to do it.