While it's true that people should lift a heavy weight that challenges their muscles, you don't have to lift per se to get a good strength training workout.
You'd be surprized how effective body weight exercises as well as a resistance band workout can be. So, if your doctor has put a limit on the amount of weight you use, then use a combination of body weight exercises, resistance bands AND the weights you can use.
If you're not sure how to use resistance bands, I highly recommend joining SERGEANTMAJORs body weight and resistance band team. he's got a bunch of beginner to advanced workouts posted. www.sparkpeople.com/myspark/groups_individ
One of the best exercises you can do for your body doesn't even use weights and that's plank. Plank is a full body exercise that will work just about every muscle you have.
So, if you don't have any resistance bands, go buy a set. You can find them for cheap at any Walmart, Target, Sears or even TJ Maxx. I use resistance bands are part of my workouts. I know several body builders who use them too.