NPC guests can actually be rude to both your park and the environment, by carelessly throwing trash onto the ground.

Here's what they look like:

Who wants that?

Fortunately, there's a way to combat the litter problem. To clean up the trash, just click that gray dirty pile and spin yourself on the floor (literally). This can be time-consuming, however, as you can only clean up one trash at a time. (Call your best friend if you need to and provide them building rights.) Another way is to destroy the path with trash on it and rebuild it. A good tip is to close the park before you prepare to clean the floor, so you don't have to be annoyed by the incoming piles of litter.

Why deal with the litter problem? The sight of the trash can even scare away NPC guests (yes, they can hate litter!), reducing their happiness and probably the park rating. Getting rid of trash can improve the rating.

To prevent this icky situation, place a trash can about 3 to 4 paths away from each food stall (or spam the cans nearly everywhere, 3-4 paths apart, to ensure squeaky cleanliness). When a guest sees a trash can, they will zoom right up and dump the litter in, instead of dropping it to the ground. When both cleaning up and placing trash cans, use trash as a guideline for where to put the trashcans.

Hopes this saves your stomach (and the guests')!