Thanks for all of these suggestions...I want to go to ALL of them.
I always want to know how things turn out so here is our plan. My husband of 20 years tells me he loves museums...I've never heard this before. Based on this piece of infomation, it was clear to me that we had to go to DC. We have already made our schedule of museum visits and are really, really looking forward to it.
But please don't feel your suggestions are wasted - I'm keeping this list for our NEXT vacation.
Sounds like you have it figured out but here are my two cents - obviously hit up the amazing free ones but the Newseum ($) is great. You receive a 2 day pass and there is a lot to see.
Also, Udvar-Hazy Air & Space (out by Dulles airport, IAD) has all the large vehicles that wouldn't fit onto the National Mall (space shuttle, Concorde, SR-71 Blackbird, etc.) - it's free as well but the parking lot there is not. If you want to save $15, you can park at the Chick Fil A or Sully Highlands Park and walk it, it's just over a mile.
Library of Congress is underrated (and I'm not talking about peering into the carrels behind the glass, basically everything BESIDES that), Spy Museum is decent if you are into that, the Lincoln Assassination Walking Tour from Ford's Theater is GREAT.
Oh, and if the planes are flying in from the N/NW at Reagan Airport (DCA) (like coming from the Arlington, Georgetown direction), I recommend going to Gravelly Point as the sun is going down to watch a few of them. It's right off the Mt. Vernon Trail just north of the airport. There is a parking lot. Don't stand on the trail though! It's an active trail for many pedestrians and cyclists.
A simplistic sample 4 day itinerary:
Day A: best of the Nat'l Mall Smithsonians (for me this would be the big 3 of Air & Space, American History, and Natural History, that's a full day but consider American Indian, African American, Botanical Garden, Art, etc. as your preferences dictate)
Day B: Memorials (mainly Nat'l Mall + Arlington Cemetery, FDR is my personal fave, don't forget about Einstein, he's right across street from Vietnam Wall)
Day C: best of rest near Capitol (SCOTUS, LoC, Newseum, Archives, Postal Museum, etc.)
Day D: best of rest near Washington Monument (Holocaust Museum, Bureau of Printing/Engraving, White House, etc.)
If you are driving from the Midwest and I-70 is your main route, avoid the toll in PA on I-76 and drop down on I-79 S when you get to Washington, PA and hit up I-68 when you get to Morgantown, WV. You meet back up with I-70 Maryland before splitting off to go to DC on I-270 in Frederick. All non-tolled interstate and doesn't cost you any time. It works when headed west as well.
Have a great trip!