Independence Hall is special because both the Declaration of Independence and the U.S. Constitution were signed there. It is currently a National Historical Park thanks to those signings.
It is located at 520 Chestnut St, Philadelphia, PA 19106. Normally, in January and February, along with after 5 pm during summer hours, it is a walk in. The rest of the time you are requiered to get a timed ticket, which are free but there is a limited number.
Important places to visit in the building are...
To learn more about Independence Hall, go to the link.