Agreed, Adams Morgan is nasty on Fri and Sat. People vomiting in the street etc. Roadies ride at Hain's Point at noon during the week, frequently in a fast rotating paceline. I doubt they would welcome you into the paceline on a fixie, but it's always a great loop to ride--day or night: I've ridden down there in the middle of the night many times. There is nearly always some wind though.

Custis Trail, then W&OD *way* out into Virginia is nice. Watch out for morons, particularly morons with dogs and/or small children. Cap Crescent is usually crowded in nice weather. Beach drive out Rock Creek into MD is nice because it's closed to cars in DC on the weekends. The mount vernon trail is good: less crowded than W&OD, but it's shorter: about 45 miles out and back from Dupont. A lot of roadies ride River Road through Potomac to Poolesville. I've done it myself many times on a road bike, but not on a track bike. To and from Poolesville is 80 miles. It's kinda hilly, and the cars are hauling ass. People die on River Road.

Get a gyro at Astor next to Bedrock in Adams Morgan.