Some thoughts on the Washington Metro System. The fare system can be a bit confusing to the uninitiated. Unlike some systems like New York and Boston were a ride is the same price no matter were you are going to, the DC metro charges you according to how far you've traveled. You price will also vary depending on the time of day your board and exit. This can introduce a lot of variables into the system. If you plan to make more then three stops during the course of a single day and don't want to do a lot of math, you're probably better off buying a day pass for the relatively modest price of $7.80.
On the positive side, trains are frequent even in off peak times, so you're not stuck waiting around on a platform for too long, and the trains pretty much travel to where you want.