I may already have mentioned this idea on the blog, but I'm getting so old now, I can't remember. And it's such a good money-spinner that I think it's worth repeating in any case.
If you go to Tesco, they have those coin-operated trolleys that are infuriatingly secured to one another with chains that you insert into slots. There's usually two minutes of swearing as you try to disentangle one from another. In fact, people will do anything to avoid battling with the mechanism - particularly when they're returning the trolley after loading up their motor.
What often happens is that people do an informal deal. Instead of reinstalling their trolley in the park and retrieving their pound coin, they simply hand the vehicle to another shopper who's just arrived in exchange for a coin. Perhaps you've done this yourself?
Anyway, here's the clever bit. Tesco trolleys take both euros and pounds. So here's what you do. You insert a euro in the slot when you arrive and do your weekly shop. When you need to return your trolley, smile sweetly at someone who's just arriving. They give you a pound and walk away with the trolley. You pocket the difference between the euro and sterling, which at the current exchange rate is approximately 31p.
It doesn't sound like much. But if you went to Tesco twice a week, you'd make over £30 a year.