Plank variation.

Bring up your knee to your chest then rotate your knee across your body. If that's too easy, extend the leg when you twist, if that's too easy do it on one arm.

I have more but I don't really know know to describe them properly.
I'll try and get a vid.
Sit facing a wall. Either extend your legs so your feet are on the wall or bend your knees. You are balancing on your butt with your feet up the wall. Put your hands behind your knees or on your thighs and sit up now hold it there. Try twisting as if doing crunch twists - take your hands off to work harder etc.
Doesn't seem like much to begin with but it burns


Watch the clip

Amazon.com: Exhale: Core Fusion Body Sculpt: Fred DeVito, Elisabeth Halfpapp, James Wvinner: Movies & TV
See 0.36-0.38
0.40 is more of a yoga pose
0.49-51