You can put a rubber band on the sink hose so that when it turns on it will spray someone. You can do the menthol in a bottle of diet coke prank, you can look that one up. It's awesome. Paint a bar of soap in clear nail polish. Put plastic wrap over the toilet bowl, or across the doorway. Hang a fake spider in the door from a thin thread. Reset all the clocks in the house to a strange hour while everyone is sleeping, set someones alarm clock to go off in the middle of the night on high volume, then hide it somewhere, like deep in a book bag or something up against the plug in the wall.
If you want to prank a whole building, release three wild animals (preferably greased pigs) into the building and label them "1" "2" and "4". They will be searching for the animal with the label "3" for hours before they realize that there were only 3 animals! It would be hilarious!