The Worldgate Athletic Club (Herndon, not far from IAD) used to sell day passes, which I used some years ago after arriving on a red eye from the west coast, when I had to freshen up for business in the Dulles corridor before heading home in DC. It was a great way to relax (a few laps in the pool, use some of the weight machines, then the sauna or steam room. But, I just checked the website and there's no mention of day passes any more, so guess that's no longer an option.