The much overlooked Ab Wheel. That $5 double or single wheel thing that looks like a toy. It will hit the lower abs as well as the upper abs, very, very well. It is a superior cord exercise when done right. And will probably be one of the few pieces of ab equipment you will ever need.
Go easy at first, try a few sets with the knees on the floor and fully stretch out until the arms are fully extended. As you get stronger, try touching the chin or nose to the floor each rep. As you advance, start with the toes on the floor, rather than the knees. And when your super, super advanced, start while standing and bend over at the waist. At that point you will discovery that the Ab Wheel is a outstanding piece of equipment. You could buy a 250 to 500 dollar ab machine, but there would really be no point in doing that. Your going to get more bang for you buck from that $5 ab Wheel investment.
Try on a smooth surface at first. To make it harder, place on a rug and from than, on a extra thick rug. Good Luck.