A great exercise for building leg strength is the leg press, especially if a more demanding lift (such as the squat) places too much strain on your body. This is essentially a squat workout done while seated on a machine where you place your feet on a platform and push outward to raise a stack of weights.
You can do three sets of leg presses, performing 12 repetitions in the first set, then 10 in the second and then eight repetitions in the last, getting progressively heavier (10-15%) each time. Allow three minutes rest between sets.
If you haven't done the leg press before, you want to begin with a starting weight that will challenge but not overwhelm you. The principle is that you want to give your legs some work in the first set but not completely fry them, leaving yourself unable to complete the second and third set.
To determine the right starting weight for your exercise, a good rule of thumb is to start with an amount roughly equal to your body weight. If you can do this twelve times easily, bump up the weight by 10-15%. If it is already too difficult, trim it down by the same amount, making these adjustments until you get to that good starting point.