Bouncing
Rebounding is a great low impact, knee friendly cardio workout, plus regular rebounding actually helps with your body's lymphatic drainage. It is also great for strengthening your core and pelvic floor!
Rebounding is also great for combatting stress, and doubly so when done outdoors.
As it is skill based, it has the added bonus of helping with your mind and body connection, your proprioception and combatting hormone induced brain fog.