"If your cashier or waiter/waitress has nail polish on, compliment it! They put clear effort into that part of their appearance, possibly even paid money for it, a short friendly comment on it is easy and typically welcome.
Otherwise, a good rule is to compliment a choice a stranger made about their appearance, not something they can’t (easily) change. Her sick winged eyeliner, not her pretty brown eyes. His hair style, not his red hair."