You really don't need a gym membership in DC during the summer. In addition to all of the free running and cycling groups that go year round there are opportunities for free outdoor yoga and I recently found free bootcamp style classes. For August and September every Wednesday at 5:30 Downtown DC Business Improvement District is hosting a workout in Franklin Park.
Trainers from Crunch Fitness lead the classes while City Sports and Freshii provide free give aways, fruit, and water.
Last week's was canceled due to bad weather, but the first two classes I attended were intense. Constant movements from burpees to pushups to bear crawls to jump lunges... After the second class I was sore for three days.
If you work downtown and can make it to the park by 5:30 I highly recommend checking these classes out. Oh and if you don't like rolling in the dirt, I highly recommend bringing a mat. Check out the website for more information. The only downside (upside?) is apparently they livestreamed last weeks workout and you might just end up on their website looking like a badass.