There is an alternative to the mini stepper that is cheaper, more portable, and more versatile. It can be used indoors and stowed away easily, like a stair stepper. That alternative is called the aerobic step.
You have seen steps if you have ever been to the gym. They are rectangular plastic structures. They usually have removable feet, which you can use to raise the step and get a more intense workout.
At its most simple, the step is just a surface that you step onto and then down from, repeatedly, to increase your heart rate. This builds your cardiovascular strength and endurance, while also giving your leg muscles a workout.
Unlike with a mini stepper, there are almost endless variations of exercises you can perform with a fixed aerobic step. You can go from side to side or front to back on one. You can do kicks or incorporate light dumbbells. You can step up and then do lunges or squats on a step, getting more than one exercise in one repetition. You can even safely do plyometric box jumps with a step.
The main reason to stay away from an aerobic step is joint pain. The step is medium to high impact, not as low impact as a mini stepper. If you doctor clears you for it, and you can learn how to keep the impact lower when needed, an aerobic step could be a great option for you.