Excellent advice, Lucky. Connecting with your audience, wherever you play is essential to a good performance. And playing songs 'kinda like' the melody is very difficult on any listener. Phrasing your songs properly is one of the most important things you can do.
I have found that with my CD, it makes the rounds throughout the facility from office to kitchen, nurses station to dining room. If they like you, they'll have listened to you a few times before you get there... and they haven't heard enough of you or the songs to be tired of you at all.
As for volunteering your time, make sure you let them know that this is a limited offer, as many facilities will bleed you dry if you are free and good.
And lastly, be good or don't go out there.