The National Air and Space Museum on the National Mall is great, but tends to be a bit too crowded and filled with toddlers sprinting around. The NASM Udvar-Hazy Center out in Virginia on the other hand is a huge facility with some incredibly awesome aircraft on display, and a much calmer space to sketch about.
You could return there a dozen times and never be lacking for a new interesting, fascinating, and awe-inspiring subjects to sketch, from the Discovery shuttle, WWII planes, aircraft from the birth of flight and every era of flight you can imagine.
And the conservation facility shows a bit of the process involved in restoring and maintaining the amazing pieces at the museum.
