Pistol squats are the best lower body bodyweight exercise you can do and are great for knee rehab. If you don't have access to a weight room you should definitely be doing these or progressing to do them.
They can be a little challenging at first and some may need to build more leg strength with squat and lunge variations, but for many it is purely a co-ordination issue.
Here are some great tips from Franz Snidesman on progressing to a full range pistol squat:
For further tips in progressions I would recommend checking out the BeastSkills tutorial.