Man, I could write a book... The best prank at a restaurant I was ever part of was when we put salt in the sugar dispenser next to the staff coffee machine. Simple, but VERY effective.
Best firehall prank would have to go to a former Toronto Maple Leaf turned FF who I have worked with. He had the other guys in the hall all get in on it to convince the new rookie that he had to quit the NHL because of a motor bike accident that cost him his leg. He never spoke about it, but the guys all quietly would whisper things like, "Man, he's amazing at his job considering that accident", and "have you seen the guy play recently? He just started up with hockey again and he skates circles around the guys, even with the prosthetic". All the while, he's take his boots off...well, the right one, and keep the left on at all times. Sometimes he'd fake a small limp and rub just below his knee, while NEVER mentioning a thing. Finally, one day the rookie got curious and asked the guys about it. Of course their response was that he'd lost his leg in a bike accident, and was pretty sensitive about it. A few months later, Christmas came around and the ex-hockey player brought in some pictures of the family playing with their presents, etc. one of the pics had been photoshopped so that he only ha one leg, and was in pajama shorts. Crutches were leanin up against the wall beside him, to complete the effect. The guys all looked at the pics, and when they got to the rookie, his eyes bulged out of his head...but he stayed quiet. Fast forward three more months, and not another word was said. The act was kept up until one day, the hockey player snapped, looked at the rookie and said, "Holy sh!t! I can't do it anymore. Do you really think they'd let a guy with one leg be a firefighter? I haven't been able to kick bot my boots off, for 8 months because of you! Do you know how hard it was to time my showers so you wouldn't see?"
Talk about commitment to a joke!