Is your brother with or without car? Also where are they staying? If you're talking only Downtown, I'd recommend the Zoo, the Air and Space Museum, the Natural History Museum, Eastern Market, and the Botanical Gardens.

If you have a car, go see the Udvar-Hazy center. Its truly amazing.

The museums are walkable as long as you stay within a range of museums. Going from Natural History on 12th to the American Indian museum on 5th is a haul (same thing for the Botanical gardens).