I once asked my doctor what would be the best home exercise equipment to get if I wanted to exercise more and wanted to do it inside. Weight machine? Elliptical trainer? Rowing machine? Stationary bike? Treadmill? Something else?

He said treadmill, because in his experience walking on a treadmill is the only indoor exercise that he's seen a good fraction of people actually stick with long enough to do any good. With all the others people do it for a while and then stop.