The truth is always the best reason. Attending to a family matter, doctor's appointments, personal matters are generally acceptable provided you have a good relationship with your employer.
How about actually being sick? Otherwise, you're stealing from your employer. Plus, if you use it playing hooky, you won't have it when you're REALLY sick.
You should actually be sick if you're going to call out sick from work. There is always a chance your boss will find out if you lie. Good excuses are migraines, the flu, or food poisoning.
