The Bureau of Engraving and Printing, also known as the U.S. Mint, is open for public tours to see how currency is made. The sheer quantity of money flying through the printing machines is incredible, literally hundred dollar bills stretching on for miles.
A tour at the mint is also quite informational as well entertaining. Learn the average lifespan of a dollar or a penny or different facts about the fine print on all the different notes. It pays to be early though, only a limited number of tours are given each day and all the tickets are usually gone by 10:00 am.