I'll post this answer, detailing the pranks in the question, sort of, as @Wikis said, to give an idea what I'm looking for.
When the actors went for lunch, LN would get on his bike, rush to the front, and get his food while the others were still waiting, because he had to have his makeup job done soon. One day, when lunch was called, he couldn't find it. So he yelled, "Hey guys, give me my bike!" and they gave it back. The next day, it was missing again, but this time... it was hanging from the rafters. And he went crazy, looking for the bike, and then everyone was laughing and looking above his head, and there it was - dangling just out of reach.
The next day, William Shatner buyers a heavy chain, and a really strong lock... and locks up the bike. And so Leonard starts screaming "Who did this?!" at which point Bill walks him to the lunch place.
Next day, Nimoy bring chain cutters, and gets the bike loose. 45 minutes later... It's gone. Bill put it in his dressing room... with some... Uh.... Territorial dogs.
I have to insert Leonard Nimoy's comment here:
Bill's dogs are meaner than he is, and that's not easy.
Next day... Leo brings his bike, in his Buick, and leaves the bike in the car. He says it best here:
Guess who had my car towed away.
As an aside, William Shatner actually still had the bike at the time of his writing his Star Trek Memories, which is where I read the story.
The pants salesman
There had been for a while, going around the set, a salesman determined to sell a pair of pants. Apparently, he was really driving them crazy.
And then... for one episode, they needed to have a giant, green, android-like creature. In William Shatner's words:
Several days later, Cassidy sat down in Phillips' undersized makeup chair and allowed the artist to transform him from a smiling young actor into an evil, hulking monster. First Freddie covered Ted's head with a latex skinhead wig; then he applied a sort of grayish-green base coat over Cassidy's entire face. Once all that was done, Phillips darkened the area around each of the actor's eyes and employed a black grease pencil to sharpen the angles of Ted's cheekbones, forehead, and chin. The end result was quite frightening and really served to drain all the humanity from Cassidy's face. In short, he looked like an android.
Star Trek Memories by William Shatner (pp 143/144)
Then Gene had a slightly evil idea. He took this evil green creature, gave it his phone, sat it in his director's chair in the office, and told the guy who was selling pants to come in. (Gene, Shatner, and some other people were hiding behind the office door.)
The unsuspecting guy walked in. The green monster holds up a hand, signals to wait a minute, says something in the phone. Turns to the pants guy. ...and amazingly, the pants guy doesn't freak out. Instead, he tries to sell a suit to the big green monster in front of him.
Gene, Shatner, and the other people can't take it any longer, so they burst out laughing and come out of hiding. And, whaddya know, the pants guy isn't annoyed - instead, he tries to sell all of them suits. (And Gene and Shatner feel guilty over the joke so they each buy some pants.)