Things on the Mall you can easily see:
Library of Congress. This is a very quick stop - the inside is beautiful, and the rotunda is a must-see.
US Botanical Garden. If you like botanical gardens.
Air & Space. The place is huge, and I could spend all day in there. The lobby has the highlights. Check out a map, and head to the top 2 or 3 sections your husband needs to see.
National Gallery of Art. I once had 25 minutes to burn while cutting through this on my way to meeting a friend. Deciding to linger, I asked the nearest docent “if you had 20 minutes, what would be the top ‘must-sees’ in this museum?” After she rolled her eyes completely around, she managed to find a pamphlet that had a list of their prized treasures. DaVinci, Van Gogh, Rembrandt, Picasso, Gauguin, etc. - you can get your culture on very quickly here.
National Archives. See the Declaration of Independence and the Constitution! (If they’re open on Sunday.) (Also, there are lines - so you may want to judge that on your own when you get there.)
National Museum of US History. There are some really amazing exhibits in here, and you could spend a gigantic amount of time going through it. If you’re a sentimental type who really likes the idea of seeing Old Glory (as in, the actual Old Glory), it’s worth the effort.
Monuments. I’m not sure if you were following along with your DC map, but I went from east to west with my suggestions. At this point, all that’s left are monuments - all of which are great to visit. I’ve never done the Washington Monument (lines), but if you head south, you can swing by the Jefferson Memorial (and the tital basin), the FDR Memorial, Lincoln Memorial and the Vietnam War Memorial (and all the “new” war monuments they’ve put up - the Korean one is my favorite). Warning: if you’re at the Jefferson Memorial and thinking of heading to the FDR and Lincoln, if you go straight there you’re going to experience the FDR Memorial backwards. Watching the nation sink into the depression after WWII is jarring, but I guess it’s nice to see Roosevelt regain the use of his legs.
I do like finishing on the Lincoln Memorial. It’s a powerful place.