I don't get the issue with traffic. I mean, 90% of the drivers here are also NOT from here....oh, wait, that IS the problem! :)
Seriously, though, I tell most to go ahead and use the Metro, if you like mass transit. I prefer a little extra cost and parking hassle to keep my own vehicle close by. There's parking, you just have to look for it.
I don't mind DC traffic as long as it's not a total grid-lock parking lot, but that rarely happens except for rush hour. Middle of the day you can get around just fine all around the beltway in nearly any vehicle. It is busy, though. Just don't try to be anywhere inside the beltway before 9AM and either head home around 3PM, or find something to occupy you 'till after 6 when traffic lightens up to leave.
Most aren't aware that many Metro stations usually have plenty of parking. From my experience with Metro, the full lots are the farthest out lots the draw people from beyond the end of that line. I work near a metro station and occasionally use the MARC rail line when my private transport gets messed up for a day - both always have parking when I go through.
For attractions, I would say you could easily spend a day seeing the major monuments. Another day each for any of the major museums that interest you.
The new Air and Space Museum - Udvar Hazy Center is a great day trip. It's out away from the city and easy to get in and out of as well as easy parking.
The Marine Corp museum at Quantico is a good one, too.
There are some off the wall places, too, which most people don't know about, like the National Museum of Health and Medicine at Walter Reed Forest Glen annex. Free to visit, parking is easy, but may be a tight squeeze for big-rigs.
http://www.medicalmuseum.mil/index.cfm?p=exhibits.index
That's just about a block away from the National Park Seminary, a group of historic buildings.
The Washington National Cathedral is another point you might not think of. Easy to spend a half day there.