Elbows on swiss ball. Rotate ball slowly in one direction for a few reps then switch directions. I call it "stirring the pot"

Assume plank position at low pulley machine. Use one arm to do rows. This is a variation on some of the other planks already suggested.

Ankles on swiss ball, elbows on floor. Jacknife up (raise butt in air). Return to plank.

Assume side plank position. Take lightweight dumbbell in top hand. Raise, like a lateral raise, and return dumbbell to front (to floor in front of hips.) Repeat, raising dumbell and return to back (in back at low back.) Keep switching back & forth. I like this because you hit delts and core in the same exercise.