This has really been done well so let me take a shot at some of the Civil War stuff. First In and Around DC there is so much “hidden” Civil War Stuff and in your situation I would tend toward looking first around town.
Let me say Fredericksburg is DEFINITELY worth it if you are interested in the CW. Having said that however, for outoftowners with maybe one shot at a Battlefield it would not be my first choice. Especially considering that with an extra 30minutes to an hour in the car you could go to much cooler places.
Total Disclosure I Used Mapquest as my time-keeper and City to City DC start, so FWIW:
Fredericksburg is about an hour and a minute away …
Gettysburg is about an Hour and 37 minutes away …
Antietam is about an Hour and 33 minutes away …
Harpers Ferry is inside of 2 hours too
All of the latter three, IMO and YMMV always, I would recommend seeing (if you haven’t) before Fredericksburg.
I second checking out the Udvar-Hazy museum. It is outstanding. You could do it before catching your flight. But another option I wanted to mention is that they run a shuttle (bus) between the Air and Space museum downtown, and the Udvar-Hazy. I think it runs every hour, and is about $9 roundtrip (I think). So on the day you’re down in the mall area, you could also take the shuttle to visit the Udvar-Hazy. Much easier than driving to and fro on your own.
If you have enough time, you could visit Udvar Hazy on your way to Dulles. But you need to consider 1) check in time, 2) security line, 3) shuttle bus to gate time when dealing with Dulles. I have to say that their security line moves pretty fast, but it can get long. I think the Udvar Hazy is definitely worth the visit, and I wouldn’t skimp on the amount of time you spend there (just how often do you get to see a space shuttle ?)
Another museum that I didn’t see mentioned is the International Spy Museum near the mall. It is a couple blocks off the mall, but well within walking distance of one of the Metro stations. When I visited it, I underestimated just how big it was and didn’t allow enough time. So do plan on spending a good couple hours if you do visit this museum. There are some really fascinating exhibits, and pretty much covers the history of espionage.
Yeah, another thing about Dulles airport is that traffic can be a real clusterfuck, if you’re traveling on a weekday. It starts getting bad on 66W around 3 pm and doesn’t let up till about 8pm. Security at the airport can be extremely erratic and then within the airport you have to deal with the shuttle bus system.
For that reason I would consider the shuttle from the mall to Udvar-Hazy rather than going on your way to the airport.
Baby Cheetahs! Baby Prehensile-tailed porcupine (probably near full grown by the time you get here)! Baby red pandas (not out at the moment)! (Not so) Baby Tigers! Pygmy Hippos (did you know there were pygmy hippos in the world?)!
Uuuuummmmm…
Zoo! (Skip the Elephant House, but do the Elephant yard.)
Go to Amazonia. It’s got fish larger than Ginger’s oldest child, free-ish ranging monkeys, birds and frogs, fresh water rays (black with yellow spots!), and to top it off, a whole hell of a lot of things to see in the Science Center. It’s a really good way to get a sense of the research behind the scenes at a zoo.
Go see the maned wolves. They smell bad, but they’re very pretty. Also, see the scimitar-horned oryx, very elegent antelope. Go early in the morning and hear the lion roar.
FREE! (Maps cost a dollar, but there are lots of signs if you don’t want one.)
I second the Holocaust Museum. When I went, you had to have tickets in advance, so you might want to make reservations, but the concierge squeezed us in at the end of the day. 100% worthwhile, but quite sad.
I also loved the Hirshhorn Museum and Sculpture Garden. It’s modern and contemporary art, which I wasn’t sure I’d like, but it was awesome. Maybe it was just a good fit with whatever exhibits they had at that time, but it was my favorite part of the trip to DC I took a few years ago.