How about renting a bus and doing a wine tour in Virginia? 'Course for folks from the Bay Area that may be anticlimactic.

You could always start at a restaurant and commence a pub crawl. The patio bar at Sequoia or Tommy & Joe's on the Georgetown waterfront could be nice provided it's not excessively sticky outside (as it usually is in the summer). For the "DC experience" (without unreasonable cost) how 'bout the old Old Ebbitt Grill? I think the WashPost rated it "best bartenders" or some such a couple years ago. You could then pick a bar for after dinner to settle in.

Speaking strictly for myself, had my parents flown out to New York for my 21st I would have opted for a smaller, more private celebration with them (as I only saw them a few times a year); 21st birthdays with the big group of friends were typically organized as a pub crawl.