I've had (4) spine surgeries and a cervical and lumbar spine fusions. Sit-ups are out - what is best for core strengthening?

Water aerobics. Swimming and water aerobics would be a good start to a healthier and stronger body.
Plank exercises. Definitely obtain clearance from your spine care team prior to starting any new exercise program so as to tailor a regimen specific to your circumstances. Planks are a varied series of static hold isometric (muscles contracted and held a steady length) exercises that can help strengthen the core muscles of your back and abdomen that can help with underlying back pain issues.
Spine surgery. Isometric exercises such as planks are excellent to strengthen neck and back muscles. It is important to strengthen abdominal muscles in combination with back muscles to give good balance. A personal trainer, pilates, yoga, a low impact exercise program and recommended stretching exercises from your surgeon will help slow the progression of adjacent level spinal arthritis.