Gym - Closest one is VIDA on 15th & P. Upscale "luxury" gym with plenty of equipment/space in the non-peak times, and good eye candy if that's your deal too
Club - Best dance club in DC is Town, though friday nights cater to a younger crowd. Sat is 21+ only.
Also might wanna check out Cobalt (17th & R), and the bar below it 30 Degrees. The restaurant below the both of those (Level One) has a fantastic dinner menu and a really good "disco" brunch (w/unlimited mimosas + bloody mary's) as well. JR's is popular as well, just down the street (17th & P)
If you're into cold desserts (skip the dessert @ Level One, it's shit), go for gelato or sorbetto at Pitango Gelato (around the corner from Vida). Or, just over by JR's is Mr. Yogato, they serve yummy frozen yogurt in a really kooky atmosphere (and it smells nice!)
Hope this helps.