Columbia Island Marina - you gotta get to it going east bound (towards reagan national airport) on the GW parkway - but it's basically just west of the 395 bridge on the potomac. (if you're coming from west bound, you have to turn around after arlington cemetary to head the other direction on the GW parkway)
Free parking, bathrooms, picnic tables, and you can put in your kayaks for free. It puts you in the marina just in front of the Pentagon, if you head up river, you can go by the Watergate, Georgetown, and get into the cliffs. If you head down river, you can go up around Haines point, head into the waterfront for lunch at the fishmarket or soemthing like that.
If you head up the Anacostia - you can paddle into the Kennilworth acquatic gardens at high tide, which I've heard is beautiful and by the arboretum.