You have a lot of great suggestions here. When I work with clients that have minimal core strength or have sustained low back injuries (cleared for exercise by PCP), I start real basic. This enables me to assess the clients ability and to avoid having to correct bad habits by simply just beginning with teaching good ones. I start with supine exercises such as pelvic tilts, 90/90s (in supine, hold hip and knees at 90 degrees flexion), alternating toe taps, bilateral toe taps, and then incorporate shoulder flexion with the lower body. This basic movement is to allow the client to focus on recruiting the appropriate muscle group, while receiving tactile feedback of the lower back to the floor to maintain the proper pelvic tilt position during the execution of the exercise. When the client has mastered this I move on to the above mentioned ball exercises, planks, crunches, then continue to progress to standing exercises with multi-directional movements such as chops, upper body exercises with bosu, and kickboxing to continue to challenge the core.