If you are not a wheelchair user yourself it might be an idea to ask somebody who is what they would like. There may be age related differences. I could be on completely the wrong tack but perhaps some kind of gps / fitness setup might interest younger people, akin to what you can get for bikes (maybe a bit advanced for Arduino though).
You can actually get bicycles with sails now, that would certainly be something on a wheelchair, but I am getting carried away