Just 1 1/2 hours south of DC is the Science Museum of Virginia, located in Richmond. We absolutely loved it! Everything in this museum is hands-on, and my very active kids loved going room to room to test things out. The building itself is just beautiful, a renovated historic train station filled with tributes to local history and train replicas. The lobby hosts a Foucault Pendulum, which is fascinating to those seeing it for the first time. My kids spent quite a while being mesmerized by it.
Our favorite exhibit was the Idea Factory, a whole floor filled with physics and enginerering exhibits. Each station had a hands-on experiment with great directions, and we tested out machines, pulleys and levers, and more. My kids played for a long time with the giant construction pieces.
My kids also loved the Light & Vision area, where they had a neat exhibit with two-way mirrors that was especially fun with our set of identical twins. There were optical illusions, including an Ames room, which blew my kids' minds. There was also this great photo opp where you can stick your head on a platter:
Another favorite exhibit for us was the Live Animal Lab. We even saw a show where one of the researcher had rats playing basketball, and while they did that, he explained how they trained them and how the rats learned. My kids (and I) found it fascinating. We compared unique traits in the You're Unique exhibit, the DNA section of the Human Body area of the museum, which was also funny because of having identical twins. It tells you what percent of visitors you compare with, like how many have attached earlobes, blue eyes, or other traits. None of my pictures from the Gravity or Energy rooms came out well–things were moving too fast!
Admission is $11/adults and $10/kids for the exhibits only. IMAX Theater admission is extra, or you can purchase it along with admission for a discounted price. Keep an eye on Groupon, Living Social, and other daily deal sites for discounted tickets as well. You can also go in as a group of 10 or more (that's just 2-3 families or a homeschool group) and pay $8/person. They have free parking–no one has free parking anymore!–so that was a nice surprise.
Read about other Day Trips from DC.