Presumably you are taking the Capital Crescent to Bethesda...from there, you would probably want to continue on the Georgetown Branch trail (crushed gravel, but perfectly fine unless you are on a super-stiff road bike) to Beach Drive. Two ways of doing this: either continue across the trestle bridge to the bike path and take that south until it runs into Beach Drive, or take a right on the first street you come to on the Branch trail and take that across East-West Highway where it turns into Beach. Either way, once you are on Beach you get a nice wide road with a long fast run down to near the zoo, where the Rock Creek bike path picks up again. Just follow that down to the river and follow your nose from there. I generally rode on roads when I was still biking a lot, but one can do almost all of the ride you are considering without having to deal with cars on weekends. Enjoy it while you can, the Georgetown Branch trail is about to turn into an all above ground Metro line (including the trestle bridge).
I was planning on picking your brain on how best to get from Bethesda to Nats Park. I assume taking 14th Street down would only be advisable if I truly want to live up to my nickname - Suicidal.
