The Smithsonian Air & Space Museum is located on the Mall in Washington DC... and it's FREE. You can visit it for part of the day, then cross the Mall to some of the other museums.
We drove in after lunch; free parking along the Mall can be TIGHT. The museum closes at 5pm, but a lot of parking spots open up around 3pm. Some Government employees will work 6am-3pm to minimize the impact of rush hour. That only gives you two hours to visit things, though.
The Metro may be a better deal, though. There are lots of Metro stops in the area.
The outside of the museum has a neat example of the planets. At one end is the Sun. The next stop (it will say how many steps away) is Mercury. Everything is in proportion so you can see the size/distance between each of the planets and the sun.
The museum is full of actual planes, jets, and rockets. There are several different areas one focuses on the planets and what we know about each of them. The Albert Einstein planetarium and flight simulator are fun to see.
Lockheed Martin has an IMAX theater in the museum. If you're going to visit, check out the showings before you go so you don't miss out on a show. Smithsonian Shows
There's a dining area with McDonald's, a pizza place, and Boston Market. Most of the places are priced similar to airport dining options; don't look for a dollar menu. If you're walking on a hot day, there are lots of signs saying NO FOOD or DRINK, but we always have snacks and a bottle of water with us. You don't want to be in DC and need a bottle of water... cha-ching.
You can't visit DC without spending some time on the Mall. If you hit the Mall, the Air & Space Museum is definitely a place to make it to!!
You can see more about the museum at their web site: Smithsonian Air & Space Museum