city's outdoor pools are open. Banneker Pool at Georgia and Euclid, the closest pool for most folks in the neighborhood, is open 1-8pm weekdays (except Thursdays, when it's closed) and Saturdays and and Sundays from noon to 6 pm.
If you haven't been, the city pools are pretty great. They're free for DC residents, just show your DC drivers license, DC college ID, utility or tax bill with your DC address, or one of these things. Non-residents have to pay $7, or $4 for kids -- still not bad.
The pool is big enough that you generally have some room to swim, cavort with buddies or just relax. They do adult swim every now and then too. There's a separate baby pool as well.
Banneker is pretty big, though chairs go quickly so you'll want to bring a towel to sit on. They're also sticklers for people to shower before they get in, which is something I can support. And they don't technically allow drinks, but you can usually drink from a water bottle.
Food is a no-no, but if you want to run out for a snack, there's always McDonald's next door, and just up Georgia Avenue is Mama Chuy's, a great spot for post-pool tacos and margaritas. J. Bell's Winghouse on Euclid is also pretty tasty.