'And what not' is casual and suits informal conversations more than formal writing. We hanged out, watched some movies, and what not.
Pro Tip 2/3
Conversational Filler
Use 'and what not' to casually imply additional actions or things without listing them all. He packed clothes, snacks, and what not for the trip.
Pro Tip 3/3
List Endings
'And what not' works well at the end of a list to suggest there are more items you haven't mentioned. She bought pencils, notebooks, and what not for school.