I do some core exercises that aren't difficult, but I do feel them targeting my mid and low back:
Quadraped
Superman
Cobra
The PLANK
The quadraped, you get on your hands and knees, knees under hips arms under shoulders, and slowly and strongly lift opposing limbs straight out - in other words, lift right arm and left leg, lower, and then left arm and right leg, lower - etc. ... the slower the better.
The superman, you just lay face down, and then, clenching your toosh and slowly and carefully, lift both legs and both arms at the same time. I wouldn't do that if you feel pain, I don't, so I do, and it kills after a bit.
The cobra is the yoga pose, on your tummy, legs straight out behind you toes pointed, and with your arms next to your breasts, lift your upper body off the floor using your back at first and then coming into a curled over your butt position with your arms straight. You should feel your mid back contracting. Again, if it hurts, don't do it!
The plank - you know the plank! Get at the top of a military pushup, or on your elbows, and HOLD IT until you die, keeping your back straight.
I bet you cold google those for much better descriptions!
I have some rolls, and had some MAJOR back-boobs. Sheesh. I have the lower back indentation still, where the fat rolls were, but it's kinda sexy now if you ask me. I would like my back to stay soft looking, as hubs strongly prefers me "squishy", but I have a great spine, no problems, so I have been treating my back to some strengthening to keep it in great shape. My goal weight is a bit higher, so hopefully that will keep me looking soft.
Edited to add: Of course I also do upper back and shoulder moves, and I can FLEX back there now. But I didn't see any lower or mid back moves listed...