Like learning a new language, reading or writing, learning to play a musical instrument at an older age is extremely doable and enriching. Residents with their own instruments might consider lending it for others to learn basic note playing.
Alternatively, a very inexpensive and easy to learn instrument is the Ukulele. This is probably the easiest instrument to play. Since it only has 4 strings and no need for a pick like with a guitar, so you can easily learn simple chords and be playing in no time. The instrument itself is very small which makes it easily portable, and lightweight. Most people can actually pick up chords from the uke in as little as a few weeks, so it also proves to be an instrument you can learn quickly, and possibly advance to another instrument from.
Here is a video of what learning the Ukulele did for one retirement community.
Click here for the original idea.