I'm not very clever but DH once pulled a good prank on his boss. He listed his bosses beloved truck in the classified ads for dirt cheap, saying he was in need of quick cash. The phones at work rang off the hook that day.
The next year (same boss, who is terrified of spiders), DH caught a spider and put it in a ziploc baggie and left it on his desk. The boss yelled at him to remove it. Later that day, his boss put on his jacket to go home. He reached into his pocket to get his car keys but found a ziploc bag and freaked out.