Leave a penny, take a penny.
We have a vending machine at work. It only takes EXACT change and it's counter intuitive to what you would expect.
I am leaving a small container of change, filled with what was sitting at the bottom on my purse.
I'll keep an eye on how much money is in it every once and a while.
I posted a sign about the cup, "Take a nickel leave a nickel. Let's face it this machine eats our money, let's help each other out."
Reaction: It lasted a grand total of 10 minutes.
Someone was offended by the sign and I took it down. I believe it was the wording of the sign. For that it was a poor choice on my part.
Will I do it again?: Nope.
How much did it cost me? my pride
How much time did it take? 1 minute total
How much effort? Not much but on further refection I would rewrite the sign.