Some couples make each other different foods or show each other movies, Austria and Prussia spend hours showing each other different songs.
When everyone is asleep Prussia will steal Austria's glasses and coat and sit at the piano and practice because he wants to be as good as Austria one day.
Austria found Gilbird alone once and shit happened...stuff that no one can know about...
Prussia caught Austria listening to the death note theme song once.
Austria has a secret fantasy where Prussia dresses in a Nazi uniform and whips him until there's a fair amount of blood and then Prussia blindfolds Austria and fucks him.
After they have heck, Prussia likes to smell Austria's shirt and wear it when he sleeps.