If you youtube "quiet cardio" you'll find a bunch of cardio exercises that you can do with no equipment, no jumping and not a lot of room. These exercises are great for apartment and hotel dwellers.

https://www.youtube.com/results?search_query=quiet+cardio

I've tried them out myself and they sure get the ticker pumping. I managed to save myself from having to buy an exercise bike (which I know I'll never touch after COVID).

As for muscle mass loss, I wonder if you could increase the amount of protein in your daily diet (eggs, white meat, etc.)?