I've got a couple of stories.
In the office where I worked someone had their "over the hill" 40th birthday and some of the black balloons that were used as decorations were still lying about a couple of weeks later, but they had deflated just a little and were no longer the shiny black that they were when they were fully inflated: they were a dull almost flat black, and when I saw it, I thought, "that looks like a cannon ball." So I picked up the balloon with both hands and go over to where one of the guys was sitting, and I ask him if he wanted to look at this cannon ball I found, and as I'm saying that I sort of "heave" the black balloon toward him, but I'm moving like the thing weighed 40 pounds, and I launched it on a trajectory that would have it landing in his lap. He sprang out of the chair so fast! ... as the balloon floated gently to the floor.
I'm in Seattle and we have the occasional earthquake up here. The same fellow in the story above was really scared of earthquakes, but he didn't tell us, so I did this to him not knowing how "well" it would work: He was parked in the building's underground parking garage, getting some equipment out of his van, and I happened to come into the garage just a few moments behind him, but he didn't know I was there. (My ninja training, you know.) I realized that from where he was he couldn't see me, so I stealthily approached his van until I was on the driver's side. He was on the passenger side with the side door open. I put my hands on the van and began to rock it back and forth with increasing energy. He locked up in terror that an earthquake was going to trap him in the underground parking lot. He was paralyzed with fear for a good ten seconds. Scared the stuffin' out of him. He didn't think the joke was as funny as I did.
Another friend was going on a vacation for a couple of weeks and asked me to check his house while he was away. He lived alone and needed someone to take in the mail and water the plants, etc. He let me know that he would be coming home after midnight on the day he returned, so I set a plan into action. I took a bunch of old clothes and stuffed them with crumpled newspaper to make an impromptu mannequin, and I had a mask I had used for Halloween that made a suitable head, especially if a hat was on top of the head. I had this mannequin installed in a chair in his living room and adjusted the room lighting so it was bright enough to see that something was there, but gloomy enough so that you couldn't make out any details. The was some low level back-lighting too so that the silhouette was unmistakable - "someone" was sitting motionless in a chair in his living room. I changed the radio station that my friend had on all the time as a deterrent from NPR to some local rap station, and increased the volume so it was much louder than NPR had been, but not so loud that you could hear it from the other side of the front door. I made sure everything was in place and I went home. It took a while before he was even able to ASK me about it. I know it scared him, and scared him real bad, because to this day - more than five years later - he wont' talk about it with me. He just says, "you got me good."
One other fellow I worked with had a series of relatively minor misfortunes befall him in a short period of time, so that people started talking about how a dark cloud was following him. I decided that he needed his own "dark cloud," so I bought a batt of polyester pillow stuffing from a sewing store and some spray cans of black and grey paint. Over the weekend I make a pretty decent looking cloud, using wire coat hangers to provide some structural support. Then I made a paper sun using orange and yellow paper from the copy room. I got to work early Monday morning and installed it right over the middle of his cubicle. It was lacking only one thing - rain. Some ice from the break room provided just the right effect. He actually smiled when he saw it.