Done loads of crap of this kind through the years, but the best might be one from my secondary school years. A friend moved to another town, and when visiting him, we found out that the night lighting of the police station and the surrounding park was controlled by an easily accessible CDS cell. When it's dark outside, the light turns on, when it's bright the light turns off. Normally you set a delay on those to avoid trigging by passing cars etc, but there they had no delay, they had simply put it in a piece of pipe, with the opening pointing upwards.
Anyways, we soldered together an astable flip-flop circuit that controlled a LED. 5 seconds on, 5 seconds off, and so on. Hid it all in the pipe and plugged the opening. The result was of course that the whole police station and the park blinked in step with the flip-flop, and did so for the most of the next day until they found out why
Oh, one more. I'm the first engineer on a quite large passenger ship nowadays. One of the second mates is kind of cocky, and the perfect target for pranks. Got one of those electronic door bells with RF connection between the button and the bell. Set the signal on the bell unit to angry dog bark, and hid it in the inner roof of his cabin, right above the bed, while he was on watch. Guess you guys can figure out the rest
"Where's the fucking dog??? Anyone seen it??? I'm gonna kill it!!!"
Edit: And a last one... used to work on another ship a bunch of years ago. It was chartered out for 5 years to go in the Mediterranean with an Italian crew. We didn't hand it over before leaving a little present though, consisting of a push button, a knob, two lamps, an extremely annoying summer, a time relay, and a bunch of blind cables going to one of the cable harnesses, everything neatly installed in one of the control cabinets in the ECR.
Every other week the time relay was energized, the summer went off, and the red "ALARM" lamp was lit. To reset it, you had to turn the knob and push the button, then the green "NORMAL" lamp was lit again, and the summer stopped, for another two weeks or so, before going off again. It was labeled ABNORMALITY ALARM. We told them we had no idea what it was, but that it went off now and then, and showed them how to reset it.
Best thing, they didn't figure it out in all those years, it was still there and working when we got the ship back