The basic exercises like Bench Press, Squats, and Deadlifts are difficult for a reason. They build muscle. Don't avoid them because you're embarrassed about the poundage you're using. You will severely limit your gains. It may look good to put tons of plates on the Leg Press and do 20 partial reps, but you really aren't building any muscle.
When people tell me their legs won't grow, the first question I always ask them is, "Are you doing squats?" The answer is almost always, "No. I can't. They hurt my knees... They hurt my back... I had knee surgery and my doctor recommends against them". I then ask what they are doing. Most commonly they are doing smith machine squats. Then I watch their exercise performance and 99% of them all do the same thing. They only go down about 1/4 the way to parallel. Even with injuries or surgery you can still squat to parallel safely if you are squatting with the proper technique and starting with a weight you can handle.
Build the core of your training program around the big basic lifts and work to improve your technique and strength. This is how you'll add some quality muscle to your physique.
I do know some people who have problems with their back when they squat. Every time they squat with a weight that stimulates their legs, they injure their back. This would cause them to miss workouts for all body parts. Barbell back squats aren't an option for them. They may not be an option for you either.
Please keep in mind the following: Exercises that cause you pain and cause you to miss or limit future workouts are impeding your gains dramatically. You need to avoid such exercises and find substitutes for them.
However, you need to be honest with yourself. Are they really causing you pain or do you just not want to do them? Heavy, basic exercises like squats are difficult for one reason - THEY BUILD MUSCLE!
However, if performing such exercises truly is a problem for you, find an adequate substitute.
There are other exercises and machines that can go a long way toward replacing squats in your workout. At least you can still make some nice gains in your legs.
- 45 Degree Angled Leg Presses - These plate loaded presses are my favorite replacement exercise for squats. I honestly feel that you can make similar progress in leg development if you train them with the same intensity but, you must go to parallel.
- Front Squats - With this type of squat you can't use as much weight and are also forced to remain in a more upright position which might alleviate some of the pressure on the back.