As kids, we always did goofy crap.
We flip the switch off on the automatic doors at the grocery store and watch at least 10-15 people bang their carts into the glass.
We'd find couples' names in the phone book like, "Mike & Karen," etc., and call in the middle of the night. We'd ask, "Hi, is Karen there", and when the husband would demand to know who was calling we'd say, "Oh, crap...is this Mike?" And quickly hang up. I'm sure we caused many arguements over those many, many phone calls.
We'd take a nearly empty cigarette lighter and push it into the exhaust pipe. Then you take an extended-out wire coat hanger and push the lighter all the way until you hear it drop into the muffler. After a while of driving it heats to the point of exploding. It causes no damage at all but it creates enough of a bang to get the driver to pull over.
One time the radio DJ we called actually announced, "Happy Birthday to Hugh Jass of Columbus, GA." I don't even know if radio stations still do birthday announcements.
At family picnics (very large gatherings) my cousins and I would sprinkle the cayenne pepper we brought with us on all the various dishes.
We put grease on teachers' car tires on wet days (don't try this - its dangerous).
We'd walk through the malls pushing one of the complimentary wheel chairs with whomevers turn it was sitting in it. We'd go through the Saturday crowds and accidently dump the "handicapped" friend. We'd blame each other for the mistake quite loudly and just run away, leaving people deciding on what to do with the retarded crippled kid. After a minor attempt to help him up, the prankster would jump up and yell something off the wall like, "I'm cured" or "thank jesus," and run off to retell the story.
Swapping products was always a basic gag but it always resulted in a laugh.
Pebbles under hub caps was an oldie but goodie.
I once took those pull-string fire crackers and tied them between multiple school chairs. When the second person sat down it provided enough yank to set it off. That was a good 1st 10-15 minutes of class time killer.
I could go on and on...