Breaking them on groceries seems to be what I see the most, but that's generally older people who still use only cash I suppose. My parents will get them for ease from the bank teller and then break them on groceries for example.
MAF's OP reminded me of this one time I was at Burger King and this guy came in and kept trying to pay with hundreds. And they all were coming up marked as fakes, and he kept trying more that he had. Then he demanded a manager, which let the cashier call the police, and he was arrested. The pigs didn't believe his story about them being absolutely legit bills he just got from the bank though. Typical pig cops.