Well... I am no expert by any stretch, but I read somewhere (can't remember where to save my life) that you can put your foot under a dumbbell and lift and lower (carefully - I'm guessing you can't have too much ROM) with it on top of your foot. I've never actually tried it, and have no idea how safe it is. I think they said to really focus on the contraction of the muscle while doing it and that you'd be surprised.
Ankle weights are another option, but I don't know that they go high enough to really challenge you...
Boy, I've been a big help, huh? ;-)
Wait, I think it was called "The Best Exercises You've Never Heard Of". Hmmm... gonna have to dig around to find it. Okay, got it. Here's the excerpt:
"-This exercise is done in a standing position.
-Balance a dumbell on the top of your foot (start light to get a feel for the balance), right across the area where your toes start. This provides the best notch for the dumbell to rest in.
-Angle the handle about 15 degrees to your foot, following the angle of your toes. If you look at your bare foot, you will see that your foot doesn't run straight across at the toes but angles 15 degrees down.
-Be sure to hook your toes up in order to cradle the dumbell so it doesn't move around.
-Raise the foot off the ground with your leg straight.
-When your leg is about 20 degrees off the ground, bend your knee, lowering the dumbell. Extend back up.
-Squeeze as hard as you can at the top and hold your leg straight for a few seconds. You should feel an intense contraction in your quads when you do this.
-Hold onto something stable for balance when you first start doing these.
-This exercise works the quads at the hip (the rectus femoris muscle of the quads is a hip flexor) because you have to support your leg in the air. It also works the quads at the knee, like a regular leg extension. This combined action gives you a greater contraction in the quadriceps than you get with a machine leg extension.
-It is less stressful to the knee than machine leg extensions and allows you to work the quads in isolation without damage and with less weight.
-These can also be done with your leg pointed out to the side. By being able to change around the directions you do the extension (not to mention the inherent instability of the exercise), you increase the real-world carry-over value of this exercise."
Maybe someone with more experience can critique that. I dunno! Just quotin' from the little e-book I have.