This evening we did something we've been meaning to do since we moved to DC. We drove down to the national Mall and went for a walk by the monuments at midnight. If you have the opportunity, I highly recommend it. There's plenty of parking around there, so you don't have to worry about traipsing around the mean streets of DC in the dark (not recommended). There's enough people around so you don't have to worry about it being too desolate, but not nearly the teeming masses that swarm about the place in the daytime. Plus, in the summer, it's a lot cooler. It had been in the upper 90s the day that we went, but by the Washington Monument there was a pleasant breeze. We only walked from the Washington Monument to Lincoln and then back through the Vietnam memorial, but that's a deceptively long hike, and we had had enough by the time we got back to the car. Another bonus of going late at night is that all of the ducks and their little baby ducklets can be seen cuddled up together along the edge of the reflecting pool.
And I can finally personally attest, as I have been dying to for ages now, that the ring of asphalt surrounding the Washington Monument is just dandy for roller skating.