Yes I have thought about weights, but I have to be very carefull with them. I have degenerated disk in my back, not just one or two but almost all of them. Plus I have stenosis of the spine in the lumbar and neck both. Not to mention a bulging disk in the neck.
I have increased some of the weights i use in my calisthenics. I increased the ankle wrist weights from 5 to 10 lb. and today I started doing squats with a 40 lb weight on my sholders. That really cut down on the number of repititions (from 40 to 12)