Jumping rope is easy on the knees? I guess we don't get/need much elevation but I have been avoiding it thinking it could hurt knees (if on the heavy side).
I jump rope on two yoga mats, one on top of the other. I've done it without a couple times and noticed the very next day. FYI I am just over a 100 lbs but I also jump for 30-40 minutes, depending on the show I am watching.
The best way is to do it how a boxer would on the toes one foot at a time. Eventually you build up your muscles and your bones become dense fast enough to perform more intense moves; high knees, double up etc.