Russian twists. I love these. They are challenging and tone up your entire core. Start doing them without any weight: balance yourself so your butt is on the floor and your legs are in the air like Miss Anna in the picture, then rotate your torso so you can touch the floor on each side of your body. Once you get good at those grab a weight/can of soup/medicine ball to add some difficulty. Make sure not to bounce whatever you're using as a weight on the floor, just lightly touch.