I would have to say that spending time with them is the greatest gift of all. Depending upon their physical or mental disabilities, other things can be done by way of engaging them or even entertaining them...but your physical presence in the life of your loved ones...nothing can take the place of that.
If, by chance, you do not know the elderly...then just visiting with them, singing a song, playing some cards, taking them outside to get some fresh air...or a myriad of other things to keep them happily engaged would be wonderful. I know a neighbor who just got her standard poodle tested and certified, and she will now be taking him to hospitals and nursing homes. That is also a wonderful thing to do!