If I can help, I will. It's always fun meeting new people by being able to do something for them.
RAKs I do are picking up fallen items, giving money to those that need some, or giving directions to lost people.
As for what someone once did for me, it was just inviting me to play. On the first day of Kindergarten, I was a pretty shy kid because I didn't know anyone in class, so during recess I was just sitting in a corner by myself. One girl willfully invited me to play and I had a blast; she became my best friend and we've been friends since then.