Ask to do more work? Maybe they are not aware that you are just hanging out. And if your manager doesn't do anything about it, it's a management problem, so go

Ask to do more work? Maybe they are not aware that you are just hanging out. And if your manager doesn't do anything about it, it's a management problem, so go above him and say "Hey, I need more work. I complete everything, have asked my manager, but still I sit around".