The classic fake "this copying machine/coffee maker/etc. responds to voice commands" sign has always been one of my favorites, though I've never done it myself. Such a brilliant, little tweak with such endless hilarious consequences.
I'm laughing so much I had to stop reading it, because I'm proctoring a test.
Another thing that has me laughing a lot lately is the Macklemoore "Downtown" song, specifically the part where the lady passionately sings:
She has her arms around your waist
Balance there to keep us safe
Have you ever felt the warm embrace
Leather seat between your legs
The leather seat line totally gives me the giggles.
The best one is probably the fake paint color "You'll Never Take Me Alive Copper."
I sent this link to Mr. Robot and he's laughing to the point of tears.