What are the righteous deeds that can be done for the deceased to benefit them, without falling into bid‘ah (innovation), such as what is called the "Fifteen" and the "Forty"?

Every good deed performed by a son with the intention of drawing closer to Allah Almighty, and dedicating its reward to his deceased father, will reach him, by the permission of Allah, especially the son's righteousness and obedience to his Lord. As for the innovations prevalent in mourning ceremonies, such as the fortieth-day commemoration and others, one must be wary of them, for the Sunnah suffices for all of that.