Put a couple $20 bills in a whistle and put it on your key chain, so it's on your person.
One day, when you come across something, where it is cash-only and no ATMs to be found within a reasonable distance, you'll be SO stoked. We can't swipe credit cards everywhere, people.
I used mine to get into my niece's basketball tourney recently, and it was awesome. I've also used it as tip money for forgotten helpers (ahem, housekeeping at hotels) too which is super fun.
Bonus: don't forget to replenish upon use!