Most people can do this: take a rubber band or a piece of tape and wrap it around that thing that squirts water out from the sink. (not the fousit) i forget what its called. Then when they turn on the sink it will go all over them. Trust me its funny
You can opt to speak softly to one of your family members so that they think they are going deaf. You can get a rubber snake and place it on the dinner table or put water in a can and hang it up against a random house's door. In whatever you do, don't pull a prank that will hurt someone.
You know those little "Pop its"? Little paper things with rocks in them, they make a popping noise when you throw them on the ground or squeeze them. I like to stick them under the toilet seat, under the little nubs that touch top of the bowl. So when someone sits down, "POP!" Scares the piss outta them, but it doesn't actually hurt anyone, and the mess cleans up pretty easily.