Depends on the kid. I absolutely despise them but I had to watch my niece once. She was decent, I just left her alone in a room with a doll and she played by herself. Then I offered her a cookie.
If it's a decent child as well, put on their favorite movie. Read them a book. Sedate her. Give her a coloring book. Go play in the snow (if you have any).
If you don't mind cleaning up messes, do some finger painting or look up some other little kid crafty thing.