Another popular one on a weeknight is, The kids next door are having a party. We have special cops assigned to harass people who drive. They like to harass the drivers of fast cars, cars blasting music, cars with expired registration tabs, and the like. It is lots of fun when you pick them out of traffic for nothing more obvious than running a red light. Sometimes you get to really heap the harassment on when you find they have drugs in the car, are drunk, or have a warrant.
Some people take off running just at the sight of a police officer.
Nothing is quite as satisfying as running after them like a beagle on the scent of a bunny. When you catch them you can harass them for hours. When you can think of nothing else to do, there are books that give ideas for reasons to harass folks. They are called Codes. They all spell out all sorts of things for which you can really mess with people.