Visiting the states - it is what you have to miss out on that makes planning difficult LOL
Udvar - Hazy definitely
The Mid Atlantic at Reading,Pa is not too far from U-H ,we did not visit the Mid Atlantic museum per se but attended the great airshow/reenactor event in June (?).
Next door to that is the lovely Golden Age Air Museum at Bethel,Pa - a gorgeous grass field with lots of lovely biplanes etc.
Old Rhinebeck (NY State) is a different sort of experience but great fun if they have a flying day on.
New England Air Museum,Windsor Locks - has the gorgeous Sikorsky VS44 Flying Boat,the unique surviving Burnelli CBY -3,B29 and some rare helicopters,one of the docents came in on his day off to show us round the restoration hangar where the Burnelli was (at that time) being restored and also took us into the storage hangar.
Depending on your route - the Curtiss Museum at Hammondsport is a lovely little spot and also fairly close to the Soaring Museum at Elmira.
Over to the west
WAAAM - (Western Antique Aeroplane And Automobile Museum) at Hood River,OR is an absolute WOW - a phenomenal collection of aircraft,cars and gliders.
Erickson Collection at Madras,OR is a lovely collection of mostly airworthy a/c.
Evergreen at McMinnville,OR of course has the Spruce Goose and other lovely a/c.
Tillamook,OR is worth visiting just for being inside the huge wooden airship hangar.
Further south - Castle AFB is quite a collection with a couple of real rarities.
And depending on time of year you are visiting - it is the final ever Reno Air Races at Reno/Stead this year (september),definitely worth a visit to the pits.