Staircase
Your staircase can serve similar purpose as the treadmill at the gym. All you need to do is stand in front, step up with one foot, bring the other up stand up on the step. Step backward using the same leg. Repeat on the other side. So basically it’s a constant up-and-down but each time you are alternating your ‘lead’ foot. You can do this for as much as you can.
