Sniper mentioned most of the stuff. Check out the Jefferson Memorial if you have time; it's out of the way but is pretty cool IMO. Same deal with the FDR memorial; it's across the lake from Jefferson. Most of the war memorials are near the Lincoln Memorial as well; WWII is across the reflecting pool from it (a much longer walk than it seems, by the way), and Korea is off to the side (very cool, something I want to try to see in the winter). Not exactly sure where Vietnam is, but I think it's close by.
If you're eating at a restaurant downtown, they're expensive but, for the most part, excellent. Georgetown is another place that has a lot of restaurant options and you can go down near the Potomac if you want.
You definitely won't be able to see everything Sniper and I mentioned in one afternoon/night, so just make sure to prioritize. Personally, I'd make sure to hit up the war memorials near the Lincoln though, especially WWII and Korea, but if you're more of a museum type of guy, spend your time there instead.
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