I leave in a port city with a long beach promenade, which extends along the coast for almost 10 km, so I'm used to seeing all kinds of disabled folks with all kinds of vehicles and contraptions "promenading" (or should I say galivanting ) every day. Adult tricycles are becoming a hit. I often see a middle-aged couple, who are very brisk walkers, and whose adult daughter with Down syndrome, rides one of those bikes to keep pace. This way they all get the exercise they need.
Here's a web site, which I'm obviously not endorsing, for you to have a look at the models. I have also seen a model with two wheels and a shopping basket in the front.

http://circlecitybicycles.com/atrike.htm