Some treadmills come with resistance levers for upper-body exercise. But they may feel awkward during a long workout. Holding the handrails reduces the workout intensity. Once you have mastered the technique, keep your hands off the rails whenever possible. When walking or running on a slant, maintaining good posture with only a slight bend at the waist is best for your back.
Exercise bikes provide fairly intense workouts with little stress to the knees.
They also tend to be relatively inexpensive. Bikes that have electronic brakes adjust pedal resistance automatically to keep the workload constant as pedaling speed changes. With mechanical brakes, the work rate increases as pedaling speed increases. Thus, you can make the workout harder by either adjusting a resistance knob or pedaling faster. For a mechanically braked bike, make sure you can easily adjust resistance with just the twist of a knob.