There are several psalms that are alluded to by Jesus at the beginning of his sermon (for example, Psalm 24, Psalm 37, and Psalm 40). Given the emphasis on blessing and doing good to others, perhaps it would be good to sing a psalm that connects with that theme. Psalm 112 would then be a very appropriate song to sing. Note, it was written in the time of the old covenant and not the time of the new covenant, and thus its old covenant teaching on a link between obedience and blessing should be noted before it is sung.
o Reformed Churches New Zealand (lyrics and sheet music) (audio)
1 Seeing the crowds, he went up on the mountain, and when he sat down, his disciples came to him.