Prank 1: In high school we wrote a fake DOS for our Apple II+. It accepted commands and ran them, but occasionally would reply with a snarky message. Our teacher was not amused.
Prank 2: This was the late 1970s/early 1980s when laser printers cost many thousands of dollars, and neither me nor my high school peers had ever seen one. I found some CGI images in a computer magazine and Xeroxed them onto pin-feed paper for dot-matrix printers. I showed them to my friends and convinced them that I owned a laser printer. The pin-fed holes just added to the authenticity, since they had no idea how a real laser printer worked.
Prank 3: My parents changed checking accounts and had a whole book of unused checks. I told my father I wanted to do a prank and he agreed to write one of those checks for $600. I showed the check to one of my classmates at the beginning of the day and told him I was going to buy a computer after school, and he could come with me. When school ended and my classmate found me, I took out the check, declared I no longer wanted a computer, and ripped it up in his face. He was stunned.
Prank 4: The local library had an Atari 400 with a coin-operated TV screen ($0.25 for 15 minutes). Without the use of the screen, I wrote a simple BASIC program to emit a beep randomly every few minutes, started it running, and walked out the door.
Some 15+ years ago ThinkGeek productized this as the Annoy-a-Tron, a small magnetic circuit board which could run on a coin cell for weeks. Tuck one of these into a well-hidden place and it will dismantle the sanity of anyone spending enough time around it.
Other more refined versions exist now from a plethora of vendors, I will refrain from linking them here.