Beach Drive, both in DC and MD, is a good, bike friendly road (especially on the weekends, when the DC section is mostly closed to cars). I've also been biking along MacArthur Blvd. from Georgetown out to Glen Echo -- a nice route, especially early in the morning when there's not a lot of traffic (at least on the weekend). But definitely check out http://www.bikewashington.org/ for other suggestions, in addition to what's already been posted here.