Let's see... my last twenty bucks...
Practical: ten dollars for three gallons of gas so I can get to Disneyland.
Frivolous: eight dollars for a Disney pin (Jasmine or Tinkerbelle or Chip & Dale). It'll have to be an open edition pin because the special editions would be over ten dollars.
Other: The last two dollars would go to the first homeless person I saw standing by the road. At least it would do some good. (I could have bought myself a hot dog, I suppose, but face it... It'll take me a while to starve to death.)