get the eggs from someones fridge and hard boil them all. then put them back in the fridge as a regular egg.
buy a remote to your tv that is the very same as you have now. Every once in a while change the channel while they are watching it. You cant do it too much, just every few hours.
If yyour roommate has a dirty room, go clean it very clean while they are away. Trust me, they come home all like wtf
put food coloring in the milk so it looks like it went bad, then bet someone money you will drink it.
Go to a junk yard and get a glass window, take it to the friend you want to prank and get their window rolled down. Then throw the glass down next to it. It will look like their window was broke.
make some cookies with bastard hot chillies, put the cookies out but put "do not eat-very hot"