I sometimes amuse myself by using www.wheresgeorge.com to track dollar bills. I stamp some dollar bills myself and then use them to pay for stuff. Tomorrow, I'll be mailing a few bills to a charity in California, just to distribute a few more of them.
I went on a business trip to Abilene, Texas last week. Took about a dozen stamped bills with me. Spent some in the Dallas airport and some in Abilene. To my delight, today I received a notice saying that someone in Abilene entered the serial number of one of these dollar bills into www.wheresgeorge.com. Talk about quick results! Talk about covering a lot of ground in just a few days!
It's a fun feeling.