"¿Qué haces en las tardes?"
Translation:What do you do in the afternoons?
It is true that when something is routine you can use the singular, afternoon. Basically the each/every component that allows you to make it singular is implied. If you asked How do you spend your afternoons? Then it needs to be plural. It is not wrong in duo's sentence, although I think in the afternoon sounds better.
Your understanding is incorrect. (There is a pleasant vagueness about tardes but afternoon would be appropriate sense.) Where you are wrong is about the earliness of "buenas noches". Unlike English and French this is not meaning "Goodbye, off to bed now" but rather is also said as a greeting at the start of the (late) evening/night.