My husband hasn't been to Washington, DC in about 20 years, but one thing he remembers thinking was pretty cool as a kid was a metro station with a VERY long escalator. He described it as when you're at the bottom you can barely see a patch of light at the top. It would be fun to find this for him to see if it is as he remembers and relive something from that trip.
A strange request, to be sure, but sometimes the littlest things can make a vacation special. :) Any ideas where this might be?