When I was 14 or so, we had acces to some computers during break. I went through each and every one of them, rebooted into Safe Mode (yeah, Windows), logged in as admin with no password, and gave admin powers to my account (each student had one, at least). Then, installed a keylogger and one of those "trojaans" that let me remote terminal, keyboard and mouse control to all the PCs (I had tried telnet server, but this was soo much easier).

Then came the fun.

"Why does the start menu keep opening by itself?"
"Why is the CD tray opening and closing on its own?"
Etc.

Then I found out social media passwords like (translated from spanish) "bigdicks". Never used them, because I considered myself one of the gray hatted. I did it just for the fun.