Having a lower leg injury doesn't mean you have to stop exercising. There are plenty of workouts that don't put weight on your injured foot.
There are many upper body workouts that can be done while sitting. Do not use your knee scooter knee platform to sit, it's not design for sitting. Always use a proper exercise bench.
There are even cardio exercises that you can do, you'll find them linked at the bottom.
Just remember to be safe when practicing these exercises. Do not put any weight on your foot until your doctor or physical therapist tell you to do so.
Upper Body Exercises Guide
1A - Dumbbell Chest Press
2A - Bent-Over Seated Row
3B - Dumbbell Shoulder Press
4B - Chest Flies with a Cable
5C - Seated Lateral Raise
6C - Tricep Extensions
7E - Seated Twist with Dumbbell
8E - Bicep Curls
9E - Supine Leg lowers from Bench
If you prefer a more simple and lighter exercise, check out the beginners seated workout. If you prefer to break out a sweat, then the pilates workout (for people with lower leg injuries) is for you.
Beginners Seated Workout
Cardio Workout with Pilates