All sound great! One I didnt see you mention is what I call a "twist crunch". Its a little advanced but my personal favorite.
Sit with your legs extended in front of you. Lift your legs about 6" from the ground, keeping them straight and together. So your balancing on your butt. Hold this position and swing your legs and arms in opposite directions parallel to the ground "twisting" your abs. Im sure there is a technical name for this, but until I know it, I will affectionately refer to it as a "twist crunch"

Do you know these? If so, what are they called?