For a time you could buy U.S. savings bonds online with a credit card. So the common trick was:

1) Get a rewards credit card (Citi had at 5% rebate card at the time if you wanted cash, but there were also other rebate cards like frequent flier miles.) 2) Purchase bonds at treasurydirect.gov. (I bonds were a favorite at the time, the fixed rate was over 1%.)

The ability to buy savings bonds with a credit card was ended in 2003. http://www.treasurydirect.gov/news/faq/faq.htm