Stand with your feet hip width apart and grab something with weight to it, like a can of soup or jar of pasta sauce. Tense your stomach muscles like a fist and take your weight from one hip bone to the other and back again.
Keep your knees slightly bent and, again, your stomach tense. This in turn will not only help strengthen your core, but also provide a unique stretch in your low back area.
If you want to progress to your upper back and core, take the arms into a tick-tock position at chest level. The elbows will be slightly bent with most of the action and energy coming from your waistline. Then bend one elbow and pull back with some effort at shoulder level, straighten your back out front. Repeat. When you're ready, do the same move on the other side.
For a bonus move, count to three, lifting your knee on three while you rock high and low to really fire up your furnace.