I like lunge squats where you put one foot out in front of you and squat on that one leg leaving the other behind. Putting the front foot up on a slight step is also good. Doing leg presses are good also. I try to keep the weight down and the reps high because excess weight bothers my knees. I found that quad exercises like these have definitely improved climbing along with extra cardio. I never do leg extensions, sitting on a machine and bringing you legs up in front against resistance. I have had 2 sports medicine doctors warn against it as it causes a sheer force at the knee and there's nothing else in life calls for this motion.
I concur with the yoga suggestions for back!