Save some milk in a plastic milk carton (about an inch) for a few weeks and wait for the milk carton to release fumes inside it until the thing is ready to explode. Your best letting the explosion of milk happen outside, then ask a friend to open it for you try and disguise it with a towel and pretend you can't open it, Kaboom.

You know when you turn on the shower, people sometimes just put themselves under the shower straight away. What you can do with this is get something slimy you would normally find haunting at the bottom of a can like bake bean water or watery soup and fill that hose up as much as you can with slimy horrible stuff that will make it though the holes when they turn it on. Hey if you don't get them with it i guarantee you it will at least make the shower a mess.

Tips
Better yet save some out of date milk till it's clogged green and nasty looking lumps, then put that inside the hose and he'll smell like sour milk when he/she gets out the shower.

Spend time collecting lots of doll clothing, jeans, shorts and underwear then shove them into the washing machine so when the family decides to do a wash they'll think those are real clothes that have shrunk.

Take the end of the tobacco out of a cigarette then replace with some shavings from a matchstick and cram some of the tobacco back in then wait for a friend to light his cigarette.

You know your cheap lighters you can buy from the shop buy a few and find an ashtray then start winding the wheel firmly without sparking it into the ashtray so everytime somebody tries to put their cigarette in the ash tray it will release sparks alot like when you spark a lighter.

Fill the hair dryer full of talcum powder or flour (preferred choice) and wait for a friend or loved one to turn it on.

while at a party with your friend and his girl, steal his phone and text the words "she'll never find out" to her phone then comfortably sit back and watch the fight. The argument usually esculates the more he denies it making him look more guilty each time he does deny it.

When in the bar bet you're friend he cannot balance drinks on his hands, luckily he'll be drunk and up for anything. Now you help him balance the two pint glasses on both hands but knuckle side. Once he's balancing his drinks on the knuckle sides of his hands just walk away and chuckle.

Fill an unnoticeable amount of flour into a balloon and convince one of your friends to sit on it and burst it.

Get some blue food dye off ebay then put it in the left shoe always funnier with 1 foot instead of 2. Then you can spend the whole holiday looking at your friend with a blue foot the whole holiday.