This will probably be my last post until Sunday, since I'm working on an article I need to finish.
When I was growing up I collected coins. I bought a few, but most of my collection consisted of coins I found in change, or got from family members. Every time I saw my uncle I would always ask him if I could "check his coins". It became a running joke with us, and I always asked him that long after I stopped collecting.
I discovered a few years ago the world of U.S. paper money. I've never collected this myself, since most of the really desirable currency is really expensive, but I've seen a couple collections of it. It's filled with wonderful engraving, particularly the paper money of the late nineteenth and early twentieth century.
Those of us in the United States see $1 bills with George Washington's picture every day, but what amazed me was discovering an 1886 $1 bill with Martha Washington's picture on it.
The 1896 series of "Educational Currency" is considered to be the height of U.S.money design. The bills were issued in $1, $2 and $5 denominations. Here are the fronts of the $1 and $2 bills. For the $5 bill, and the backs of all three bills, check the Wikipedia article, which provides a good description and discussion of the meaning of the bills.. George and Martha both appear on the backs of all three bills.
This is a good summary of the history of educational currency.