Fitness with knee pain?

Im a fairly active mother of 3. I work fulltime and go to the gym as often as possible. I need to find some exercises for my legs that don't involve lunges or extention of the leg. I have had severe pain in my knees since a car accident in my late teens - I'm 34 now. My legs are really the one part of my body I feel I have no control over to train. I do leg press at the gym but other than that I am at a loss of what to do...any ideas. I have just had my 3rd child so still losing the weight from that but it is coming of steadily now so I'm happy with that. I go walking a lot but I find that hills are extremely painful. The evening following a walk with my kids its excruciating. My GP says I need to train the muscles in my legs - BUT HOW - is my question.