I don't know what basically do couples talk about on their date.My girlfriend said i know nothing about dating?

I'm 17 years old boy.when i'm with my gf i talk about things which i can even talk with my sister about .i always see couples talking with very low voice when they're on a date.i wonder what is so serious they are discussing that they don't want anybody to hear or disturb.so i'd like to hear what do you talk about when you are on a date,you can provide a list if possible except for pickup lines.PLEASE NOTE:I NEED ONLY THE TOPICS THAT U DON'T WANT ANYONE TO HEAR BY THE TIME YOU ARE ON A DATE