Fancy serial ( solid , single digit serial , ladder up and down , radar etc ) serial , Error , and type of the bill ( age , rarity etc )
Well, for starters, you can spend your collection in an emergency! All you can do with Stamps is "mail letters"...
My 10 basics to collecting U.S. paper currency. 1) Knowledge is essential. 2) Collect what you like. 3) As a buyer, practice patience. 4) Learn how to grade. 5) Develop relationships with dealers and fellow collectors. 6) Buy the nicest example you can afford. 7) Moderns are a crap shoot. 8) Ask questions. 9) Try to attend coin/currency shows. 10) Buy the note, not the holder. The significance is partially historical, and will be enjoyed even more if you are a history buff. Paper money collecting is somewhat done for the beautiful vignettes and designs that adorn some of the older large size notes.