A great BW isometric leg exercise in the
wall sit in which you place your back against the wall and sit in a half
squat position. You can make this as challenging as you want by the hold time. 60 seconds is quite tolerable but things start to burn as you hit the 2 minute mark. If you can hold for 3 minutes, you are great in my book. The wall sit is a great isometric hold exercise, one that I have incorporated in my workouts.
Try this tougher variation. Get in the wall sit position. You might need to have your feet a little further from the wall than what you normally would. Now, perform alternating leg extensions. If you are a stud, you can extend each leg and perform a hold (1 to 10 seconds, for example) before returning your foot to the floor and performing the same with the other leg.
Let me know what you think.