Today is Earth Day. Well, heck, as the price of gasoline continues to rise, every day in metro Washington is Earth Day. Transit ridership is very high and making news, in part because there's no event associated with the upturn. Good.
This post, however, is for the tourists. I know you're coming and you ought to know about a set of bus lines that are now mature enough to promote: the Circulator. Read about it. It only costs a dollar: quarters drop faster than sliding a bill, and don't bother with the parking meter/ticket machines unless you have no cash and need to use a credit card. Oh, unless you did use the ticket machine, ask for that transfer and make good use of it.
The first two services -- making a T though the tourist- and business-richest parts of Washington DC -- have been expanded becoming more useful to in-town residents and a third line now loops clockwise around the Mall and associated monuments.
This is the easiest way to get to Georgetown attractions from downtown and the only direct way to get to the hopping rich-with-local-color fish market on the waterfront. The Lincoln and Jefferson Memorials are still a jaunt, and the FDR memorial is a hike, but these monuments don't have regular bus service either. (I don't recommend the 13-series buses, which are infrequent and really meant to connect the Pentagon with downtown.) The only major Washington attraction not served by the Circulator -- for which you'll need to use a Metrobus -- is the Episcopal cathedral and the Adams-Morgan nightlife district.
For the cathedral -- which I refuse to call the National Cathedral -- take any bus in "the 30s" from the bus stop on 15th Street NW catty-corner from the McPherson Square Metro station or, if you're at Friendship Heights station, the same busses stopping in front of the Neiman-Marcus. (30-series bus map in PDF.) The Greek and one of the Russian cathedrals are in the same neighborhood. Alternately, you can take the N6 bus from Dupont Circle, but that's harder to describe. From the north subway exit, look for the bus stop near the Kossuth House -- see the Hungarian flag -- at 20th and Massachusetts NW. Ask to be let off at the Cathedral because -- believe it or not -- you can't see it from where the final stop is.
Adams-Morgan is harder: the commercial strip is 18th Street NW between U Steet and Columbia Road. You could walk from Dupont Circle, but that isn't likely for out of towners. Take a bus in the 90-series from the Woodley Park/Zoo Metro station going south. The little 98 "Adams Morgan Link" bus, near the McDonalds, is probably the easiest to understand, and is subsidized so it only costs a quarter going either way. You can pick up this bus at its other terminus, too, at the U St-Cardozo station, on the Green line. Actually, I prefer that part of U Street to Adams-Morgan anyway -- more of the "Black Washington" experience; fewer drunk kids from the 'burbs, but still very lively -- and the Ethiopian restaurants are migrating to U Street between 12th and 9th streets, and on 9th south of U.