Last weekend we went to old town Alexandria and took a self-guided walking tour. It was pretty hot out, but the day was beautiful and the tour was pretty fun. Part way through Chris had to return to the car and feed the meter, so we abandoned the tour and just walked around near the waterfront. Among the historic things we saw were Lafayette's house during the Revolutionary War and the old torpedo factory.
There are tons of restaurants on the main street and on the waterfront, as well as shops and museums to go into. It's a fun (and free) place to visit if you have some time on your hands!