Contact your county extension services and ask about Master Gardeners.
Master Gardeners are a trained volunteer group who do plant clinics, gardening workshops, creation of flower and veg gardens at senior centres, hospitals, schools, etc. They will gladly schedule something for your group, and will involve as many of the seniors as are able in the creation and maintenance of the new gardens.
Find your county extension service in the phone book under County, or online by using a search term that includes the name of your county and the words 'extension services'.
Seniors could easily tend to a container garden on a patio or along the foundation of their apt.