Looks like a couple of very interesting exhibits at Corcoran; that will be on my list. I'm not a super fan of modern art so the Hirshhorn may not make the cut. Unfortunately the end of March seems to be a fallow time for photographic exhibitions at the National Gallery.
I did visit the Torpedo Factory in Old Town a couple of years ago when in for a wedding.
I went to the Air and Space Museum when last in town, but only for a short time before a flight out of Dulles. Will have to try to get there again. I still remember the old (small) Air and Space Museum just off the mall.
I didn't know there was a National Building Museum. It does look interesting. But $22 for ticket to Newseum - would have to devote most of a day to justify given everything else in DC.