needtoheal In terms of basic toning and strengthening, for my money (does anyone ever talk like that?) it is hard to beat rear and side walking for 20 to 30 minutes at a time. Walking backwards and sideways puts a constant isometric load on your quad that you can vary with stride length to the point of actually pushing your entire body weight when you're ready, but not only that, it also works your support and balance muscle groups (think knee stabilization) with every step that you are controlling and shifting your weight from front to back and side to side, and finally, last, but not least by far, it is also one of the safest exercises you can do with a quad injury. All I can say is that it kicks my butt good every time!
Close second would probably be leg raises till you can't stand them.
@KneeToKnow...agree will all the above. I also highly recommend using a StairMaster if you have gym access. As mentioned previously, stand on the outside of the machine and use only the injured leg. I still do 30 minutes a day after two years and my injured leg was never smaller than my "good leg". Also really like the leg extension machine (solo) as long as you start at a 70 degree bend versus 90 (too much stress on joint). I apologize if some of you have heard this already.
@Jeff the Klutz...glad you are home and starting down the recovery road.