By spending time with them.
You make his or her day memorable in the following ways

many people are put in old age homes as their kids find it as a burden to take care of them... spend time with grandparents:never make them feel lonely or separated or neglected
ask them what they like to do as a hobby and join in their fun
tell them how great it is to be old that you are laid with no burdens but only rest

tell them how important they are as you can learn a lot from them
sing songs, crack jokes and make fun
cause oldage is always the 2nd childhood

hope u like these ideas...