The floor in my dorm...

[Ashe]

Some of my favorites involve freezing (completely) a bottle of whipped cream (or shaving cream) and then poking a small hole into it slowly and removing it. (Make sure to look up the exact way to do this as it can otherwise be dangerous).

Then leave it sitting around in someone's room and wait for it to thaw. It'll cover the entire room.

Another: Wrap clear plastic wrap (like saran wrap) over the toilet seats. Obviously it won't be seen and someone will either piss or shit and yeah.

Also, there is a way to use jello mix on the net to basically turn your toilet water into jello. Meaning if they poop on it, it just kinda sits there.

Pop out someone's peephole one day and turn it around so you can look in and you can't see out.

Oh, and the screenshot/hide icons trick... I used to do that all the damn time back in High School. Amazing how many teachers (including tech teachers) were just dumbfounded by what was wrong... "Let me try to reboot it again and run an antivirus"

EDIT: Step one, cut a hole in a box...