I've seen quite a few older people in my city riding around on 3 wheeled bikes, similar to the ones shown here. They offer quite a bit of stability, without making you low to the ground like a recumbent. However, I can't think of how you would get one into a car. Getting a regular bike into a car is hard enough, have you thought about a roof rack?
You almost might want to look into stabilizer wheels which can be attached to any bike, and you may be able to find some that are removable for transportation.