Since Im such a miserable mood, a few suggestions I found...
When on holiday, set your alarm clocks to go off early at high volume.
Smear fake tan all over phone receiver, and watch the colour change on your victim.
Pour powdered milk in your targets bed, when they sweat it will melt onto them and stink. The milk stays in the pores for days.
Bet your pal they cant get a pool ball into their mouth. Maybe they can get it in, but not out. The hospitals own a device for getting them out.
Using a syringe, inject garlic or chilli oil into their toothpaste.
Repeatedly pour oil under your targets car, the will think they have a leak.
Pour popcorn in their exhaust pipe, when it gets hot...it pops.
Fill targets umbrella/coat hood with hole punch waste.
Put dog poo down golf holes.
Pour bird seed on victims car. It will be covered in bird poo before long.
Put gelatine down targets toilet, it sets hard in a few days.
Write a message in the grass with weedkiller. Takes forever to grow out.
Throw meat balls into garden, followed by an empty rat poison box.
Feel free to add to, I need more idea's for those pesky kids.