Ive found £20 as a kid in the street and kept that, ive found a wallet as a 20yr old and handed it in to the campsite, only £20 in it but the person got back to me and her being grateful really made me up,
i found about £50 in a self service till in asda, handed it in, but got no thanks for it, and suspected it would be pocketed by the employee, next time, i will do as the guy above and leave my name and address. found a mobile too and handed that in, or and a fiver. kept that though.
On a simillar note, you can buy joke £50 pocket paper hankys, which when scrunched up, look like the real thing. we have great fun putting them on the pavement on break time and seeing peoples reactions, one person stopped their car, to grab it, great fun!