National Constitution Center (Philadelphia, PA)

The National Constitution Center opened in the summer of 2003 on the southern end of a quad shared with the Philadelphia Visitors Center, the Liberty Bell, and Independence Hall. At a moderate fee, entrances are timed, and start with a one man multi-media show.
The show is moving, and they take advantage of a 360-degree screen running across the top of the auditorium. The purpose of the show is to display about how the Constitution isn't something that was written in 1787 and forgotten about, it is a living document, and they touch on several ways the Constitution has been adjusted in it's history. After the show, the museum begins upstairs.
Most people will describe the Constitution as the Bill of Rights, but the Center takes it past that and goes into much detail about the main body before the Bill or Rights as well. The outer circle of the museum is about the Bill or Rights, and what those rights have meant in different points in our history. There is however a clear time continuum through the exhibit which is difficult to explain. Along this outer loop there are "Can you vote yet?" booths. It starts with simple questions asking you to choose your state, and if you don't live on the east coast you aren't a citizen yet, so no suffrage (this applies more when there are more territories in the United States. If you are on the east coast, they'll ask if you are a man, if you're white, and it you're a landowner, and some states ask if you owe taxes or are a felon or something along those lines.
The inner circle goes into great detail about the three branches of government. In these sections, you can vote for your favorite president, preside over a Supreme Court case, take a citizenship test, and even take the oath for president.

The last part though is my personal favorite. The museum has made bronze sculptures of each of the people involved in the continental congress, including those who chose not to sign the Constitution. The bronze sculptures are close to life size, and make for great pictures. My picture on this site, I was at the Constitution Center, and noticed that my cell phone would fit just about perfectly in Benjamin Franklin's outstretched hand. So, I took advantage. Makes for a great picture, especially in a city where he is revered as a town hero.

I would put aside about an hour and a half to two hours at the museum, and there is a cafe on the first floor as well as a rotating exhibit. It would fit in at any point during a day in Philadelphia's Old City. For my suggestions of a walking tour, please contact me personally and I'll gladly set you up with a map.