This is great (but evil) prank to pull on a friend, make sure they have a good sense of humour mind.
Buy a gift card from a store you need to do a little shopping at. You can make the card any amount you like. They come in amounts like £5 or £10 up-wards.
Pick up some items and pay for them using the gift card you just purchased, but leave a tiny balance of say, 10p on the card.
Wrap the card and send it to your victim...er....friend telling them it has £25 credit on it as a pressie.
Most people will not take the time to check the balance before using the card. The person will most likely think pleasant thoughts about you and figure perhaps you're not such a bad person after all.
They drop buy the store to buy a new toaster, (or similar), and then they head to the register.
Imagine your victims shocked look on their face when they go to redeem a card for £25 and find out there's only 10p left on the card. First they will have a sense of disbelief and denial the card does not work. Then when it sinks in that you pranked them, humiliation sets in.
Warning: Your friend may come looking for revenge and put you in a coma.