I live in New York and am used to paying exorbitant amounts to play tennis for a short time. Therefore every time I leave town, I try to pack my tennis gear. Washington DC did not disappoint. There seems to be several public facilities. I ended up choosing the East Potomac Tennis Center as it was the nearest to my hotel and I was able to make a reservation without a hassle.
The East Potomac tennis center is beautifully situated on the Potomac river, close to many of the monuments. There is some parking available, but this facility is also conveniently located near the Smithsonian stop of the DC metro. The weather was great and in early October, there was still natural light until 645pm or so.
The outdoor hard courts (some with lights) seem newly resurfaced and the cost was just $8/hour. There are also outdoor clay courts at $15/hour and indoor courts at about $20/hour. These are really bargain basement rates compared to New York.
The gentleman working at the front desk was accommodating and allowed us to hit for on the unlit courts as we waited for the lighted ones.
Overall, a beautiful experience.