I like planks (with extra weight), Dragon flags as suggested below, and I do a couple others that may or may not be something that others do. I hold myself up with my arms at half extension where my countertops meet at a 90. Legs straight out and I maintain that position as long as possible. 3x I bridge between two chairs arms and legs straight out and hold in a slightly elevated position as long as possible. 3x I also hang drywall... Very good for the core