The National Building Museum is a hidden treasure in Washington, DC. This extremely impressive building (which can be rented out for weddings and other affairs) is gorgeous to look at from the inside. The pillars and structure are an architectural masterpiece.
There are various exhibits in the museum, which do change every now and then. Photos aren't allowed in most of them though. The best to us was the Lego one, which showed famous buildings from around the world made out of Legos. Very impressive!
Other exhibits show visitors examples of architectural masterpieces from around the globe (but mostly America). A good 90 minutes can be spent here exploring the museum (both the building itself as well as the exhibits) and is well worth it. They also currently have mini-golf set up in one section of the building for those with little ones. There was a very long line for this though, so it's something we decided to skip.