The only acceptable response to a nod of acknowledgement is another nod. If you speak, you have broken the point of such a gesture.
(1) Walking past a friend to class
(2) Catching a friend's eye across the room to say "hi" without verbally saying it
(3) Giving approval of a friend's actions
Me: Naw I'm just gonna pregame and play some Nintendo, then go to bed.
You: What you pregaming for?
2 - Cat decided to ride her bike to school as exercise, disregarding the fact it was 10+ miles away and she needed to be there in 30 minutes.