Speaking of taking the bridge from Alexandria, if you've got an hour of free time or a bicycle, you can walk or bike from Old Town to National Harbor along the Wilson Bridge sidepath that opened last month. It runs from the Washington Street Beltway overpass along the north side of the bridge to Maryland, then crosses the Beltway and takes you to the development. It's about 2.5 miles from Washington Street to National Harbor, but it seems shorter than that--the bridge trail has three viewing platforms with telescopes set up for you to inspect the scenery (by which I certainly include planes landing or taking off from National).

This post from a local biking blog details the experience of crossing the river this way; note, however, that nobody's enforcing the ridiculous 10 mph bike speed limit, and that the signage at the National Harbor end has gotten a little better.