One stomach exercise that I have always done (with great results) is the six-inch leg rises.
What you do is lay flat on your back (on the floor). Put your arms out (like you are about to give someone a big hug); keep your feet and legs together; And then slowly, raise both legs up off the floor. Only raise them about six inches up, and then hold it there for several seconds. Do this several times and I promise that you will feel it the next day haha.