Haley and I went to the Air and Space Museum. It's also free and one of the most visited Smithsonians. (Some books say it is the most, others say second, who knows). This is one of the few museums I can remember visiting last time I was here when I was 12. It has some interesting information and it really makes you realize how incredibly far we've come in such a short amount of time. In a 100 years we went from not flying at all to having super high tech airplanes and space crafts. It really is amazing. However, this museum was incredibly stuffy and full of lots of kids all over the place making it chaotic. I think if you have kids this is a great place to come, but I was ready to leave after being there like 20 minutes. I enjoyed some of the other Smithsonians a lot more.
Even outside there were lots of people.