Ex: "what shops are you planning to go to". It's fine to answer "I don't know yet". They're just testing behaviour. If you start being defensive or aggressive, pretend to know their jobs better than they, etc, it's suspicious. Although yes, in general, the US agents are really bad at doing behaviour testing.
Anecdotal: Last year, I crossed the border a few times by car, visiting a friend I met on Tinder. I completely got away with it, giving honest answers at the border. Recently met someone else (a girl) who was stopped and accused of prostitution for doing the exact same thing. :/