The Udvar HazyCenter is the branch of the Air & Space Museum located off of Rt 28 near Dulles Airport. Parking usuallly costs $15. DH cleverly discovered (by reading the website) that parking there is free after 4:00 pm. The museum is open until 5:30 pm. An hour is just about the limit of DD's attention span right now, and we don't feel like it was a huge waste of money to only stay for an hour, since we didn't have to pay for parking.
It is a nice escape for gloomy Saturdays when we can't get out for a walk or playground activity - lots of room to roam and explore, and by that late in the day, not at all crowded. The line for the observation tower closes at 4:30 pm, but both times we have gone is has been cloudy so we haven't waited in line to go up. There has been more than enough for DD to see with all of the aircraft and related items on display.
ETA: The museum is part of the Smithsonian, so there is no entrance fee.
https://www.nasm.si.edu/udvarhazy/
Re: museum tip for NoVA ladies
I've seen cars lined up about 3:45pm along the side of the road waiting for 4:00pm. It's a great idea
Let me know next time you go. I'd like to join you
Big - 1 year old
Bigger - 6 years old
Biggest - 13 years old