The side plank is a phenomenal core exercise that is both challenging and can be very spine saving. Flexion based core exercises like situps put high loads on the spine in relation to the core muscle contraction you get from them. Done properly, are challenging yet safe for your back and can get you moving towards a more functional position like standing with a good core brace which is essential to any sport as well as daily life. An important note, especially when dealing with a spinal injury, is getting in and out of the plan position using your hips to move instead of your back. This exercise also has a built in safety in that you will fall as soon as the muscles you should be using stop working:)
The squat is a foundational movement that everyone should be able to do, but many people have trouble doing properly today. Lack of hip mobility and stability as well as core stability are common limitations, as are muscle imbalances. We take a rehabilitative view here, which can be different from a strength or power development one. The fundamental spine and hip position is not negotiable. The foot placement and rotation can be varied in properly functioning people depending on their goals and personal anatomy. A good quality functionally sound squat shows us a lot in terms of overall ability to move properly. I would rather have people do great body weight squats than less than ideal weighted squats.