ON their BASE UNIT telephone remove the connector cable, put tape on it, trim it up REALLY PERFECT...then plug it back in. It will take them the LONGEST time to figure out the problem. We used to do this to each other at work...then call the guys extension. It was GREAT watching them saying HELLO...but nobody is there!!!

Flip the batteries around in all their remotes, turn their clocks back 12 hours, hang their pictures upside down, turn off all the lights left on, turn on all the lights turned off, open the microwave door, put dirty dishes in the sink, make a fresh pot of coffee and set the timer for 7 am. Silly poltergeist stuff like that.

If you really want to mess with them fill their cupboards with food,

I put the remotes in unlikely places, reduce the toilet paper rolls down to a few squares, and mess with the thermostat settings, so each day of the week has it's own operating cycle.
Oh, and offset the alarm clock by 12 hours.

Hide the coffee, swap out the drawers in their kitchen, hide the remote

I would cook a meal for two and leave it on the table, put towels on the bathroom floor, leave an empty condom wrapper in the bedroom and muss the bed and the television would be on in the living room.

Normally, I'd say break into a bank and steal just the security cameras, but since stealing is out of the question, how about dropping fifty dollars in the juke box so it plays Barbie Girl for hours on end?

Carefully remove and switch the labels on all their canned goods

Write “I know what you did last summer “ on all their bathroom mirrors

Switch the salt and the sugar.

I’d superglue the refrigerator doors as well as the cabinets.

Or to really fuck with them go in and change their locks because they wouldn’t figure it out until they got home later that day ?

Rearrange the silverware drawer.

Post sticky notes all over the place and tell the guy who lives there, "You are the father. We're having another baby!"

Or...

Take the cooking oil out of the pantry and shine their floors with it. ?