Marty,

Find a strength/bodybuilder type person and investigate "Russian Twists". While they do twist your torso, I have found the lower back only gets tired. The core and upper legs really get a great workout. If you feel they are safe for you, make sure you get proper instruction on how to them. There is little benefit to doing them wrong; outstanding benefit for doing them right.

Stay motivated and most importantly, stay safe in the workouts.

My $.02