Some things might be on here twice, so sorry about that.
1. Pee Frisbee
The pee frisbee.
As long as there’s a gap under their door, you can get them.
You take a regular frisbee, like one you’d get at the dollar store, and you pee in it. Put it upside-down, pee in it, and put that in the freezer for a few hours. You’ll have disc of frozen piss.
Carefully remove it from the frisbee, take it to your neighbour’s house [or friend’s, or stranger’s, I don’t care, as long as you keep that piss disc cold, man], and slide it under their door.
It works best if nobody is home and won’t be home for a few hours. That pee will thaw right on their floor, and they’ll come home to a puddle of piss in their house with no sign of anybody getting in.
Who the fuck peed on my floor?
That will haunt them for years.
2. Valentine's Engagement Prank
It's that prank where, if you're a waiter or waitress at a restaurant where a couple would normally go on Valentine's day, you bring various fake engagement rings to work with you and before bringing them their drinks, you put the ring into the woman's drink and watch her get all excited while the man is very confused by what just happened. If you want to make things better just in case it gets bad or something you can go back and say you got the drinks mixed up with another table and blame how hectic valentines day is.
You can do this to several couples, if you wish.