Stephen,
Much good advice has already been given, especially regarding audience interaction. An activity director once told me she has seen many entertainers who either have good musical talent *OR* have good audience interaction, but very few who provide both. And that, IMO, is the key to being successful, you have to give them... BOTH.
Do a reality check. You can obtain an activity schedule from any senior home, check out the entertainment listings and go and observe. After doing a fair amount of "checking out the competetion", give yourself an honest rating. If you don't stack up at least average, go home and practice, practice, and practice some more.
When you're performing, have fun doing it and show your enthusiasm... that's contagious. Memorize your lyrics (you are gonna start singing, right?) so you can make eye contact and do a little shmoozing, or soft-key "flirting".
One more thing... song selection is very important, especially for a first time performance - it's your only chance to make a first impression - and you have to impress the both residents and the staff. When observing other performers, take note as to which songs are well received. Most songs should be upbeat or what I call "fun-time" music, with your closing selection being a barn-burner that'll leave them humming the tune and/or wanting more.
Good luck and keep us posted as to your progress.
Glenn