Nice, nice. DC is an awesome city. So much to do. For starters, you'd probably wanna know that August is humid as f*ck here - just a heads up. You really don't need a car in the city, there is public transportation galore. Do you know what area in DC you'll be staying in?
I used to workout at the Fitness First in DC, but for the past 6 months or so I've been heading home (Maryland) for my workouts. I don't know if that would even be a convenient location for you though, and it gets pretty crowded in there. There's a few other gyms in the city you could look into... Washington Sports Club, Gold's, Bally's... As far as supps... I know of Vitamin Shoppe and GNC, but honestly, I just buy all my supps online. Georgetown has all kinds of nice places to go shopping. Lots of upscale shops and boutiques...
A few nice restaurants I'd recommend checking out:
Zaytinya
Fogo de Chao
Equinox
Tosca
Oh yeah,
Sushi Taro has awesome sushi if you like it. (I'm a huge fan.
)
As far as nightlife... It really all depends on what you're into. There's a plethora of clubs, lounges, bars, pretty much all over DC, but without really knowing what kind of scene you're into, I wouldn't even know where to begin with recommendations. If you wanna socialize, I personally love hanging out outside at the waterfront in Georgetown (Sequoia & Tony & Joe's). Great place to go talk to women when the weather's nice, and very casual. Georgetown gets pretty packed on the weekends. You can pretty much just walk down the street and follow the hot women to see where they're heading.
There's also lots of new lounges/clubs/bars in the K Street area that have opened up in the past few years. Most of them are pretty posh and exclusive, and guys can have problems getting into a lot of spots unless they're with women. I'm pretty well connected, so I might be able to pull some strings with guest lists and what not if you're interested in checking something out.
Ummmm... I could write a lot more, but I'm not really sure what kind of things you're into, and I've been rushing this cuz I'm off work, and I've gotta go workout. I'll be around though, so feel free to hit me up anytime.