I have gone with what is basically a leaflet, but once produced it would be made of card and sealed at the opened end with one of those sticky circles.
After doing a bit of research on Rome and looking at any quirky habits they had, I came across the ritual on chucking coins into the Trevi Fountain, which actually features in my mural. There are two ways in which this custom is done, you can either go with the 'chuck 1 coin in to return to Rome, chuck 2 in to find the love of your life or chuck in 3 to either get married or divorced' OR you can go with 'chuck in 3 coins to hopefully have good luck forever'.
I have mixed the two up, by giving away 3 coins that claim to be 'lucky', (in a sense of that they will end up getting you something for free), but using the 1, 2 or 3 coin system to receive different things.
The coins would be made of a cheap metal (easy to produce) and would not have any value other than in a Yo! restaurant.