Tell someone you’ve switched the contacts in his phone around, but don’t actually do it.
If you dip cotton balls in water, they will stick to anything on a cold night. For example on a car.
Stick a small helium balloon in the toilet and close the lid. When it opens the balloon will float out and startle them, could even draw a face.
Have you and a couple of friends go up to a random person, not all at once, maybe 30 seconds apart from each other, and say things like “Wake up!”, and “You know you’re dreaming, right”.
Every time the person comes near you, look down at the floor and giggle softly, then excuse yourself. Helps if you have more people in on it as well.
If there is something new (example: coffee machine, printer) Put a sticker on there that says “Voice Activated” and the company’s logo, prepare to watch people talk to a machine and have it do nothing.
Scrape the middle out of Oreos and replace with toothpaste.
Cover a bar of soap in clear nail varnish and watch people wonder why it won’t lather.
Make some caramel onions. People won’t be able to tell the difference until they take a bite.