The questioner asks: What are the best deeds a person can do in order to please Allaah (The Most High) and (also) benefit one’s deceased parents?
Supplicating for your two parents is from the best of deeds that will benefit them after they have passed away; fulfilling their promises (or contracts) after they have passed away; keeping the ties of kinship which cannot be reached except through them and showing respect and kindness to their friends.
It has been reported in a hadeeth on the authority of Abu Usaid Maalik Bin Rabee’ah As-Saa’idee (radiyallaahu-anhu) who said: One day whilst we were with the Messenger (sallal-laahu-alayhi-wasallam) a man from Banu Salamah came and said: O Messenger of Allaah! Is there anything left by way of honouring and showing kindness towards my parents after their death? He (sallal-laahu-alayhi-wasallam) said: Supplicating for them; seeking Allaah’s forgiveness for them; fulfilling their promises (or contracts which they may have left behind) after their death; showing respect and kindness towards their friends and keeping the ties of kinship which cannot be reached except through them (i.e. their relatives). [Reported by Abu Dawud, Ahmad and Ibn Maajah]
[Source: Fadaa’il Al-A’maal pages 28-29’ by Shaikh Muhmammad Bin Abdillaah As-Subayyil (rahimahullaah)]