I've managed to build a significant amount of muscle mass just from doing stretchy-band PT exercises to deal with shoulder pain. 10-15 minutes nearly every day makes a big difference. I've also started developing some serious leg muscles just from walking up and down stairs in my 2-story house (+basement) and around my mildly hilly neighborhood.

I drive as little as possible; I went on a 1200 mile road trip last weekend and I'm still paying for so much sitting.