It is Mustahab that the one who is riding greet the one who is walking, and that the one who is standing greet the one who is sitting, and that a small group of people greet the large group of people, and that the younger one greet the older one.
It is Mustahab – except in Salat – to reply the Salam in a better manner; therefore, if someone says to you: “Salamun Alaikum” it is Mustahab to reply by saying: “Salamun Alaikum Wa Rahmatullah”.
It is Makruh for a man to say Salam to a woman, especially if it is a young woman.