I like weighted side bends.  You basically stand up straight, feet hip width or so apart, hold a dumbell in one hand and lower the dumbell towards your knee a few inches, then raise it back up.  You want to keep your shoulders in line with your hips (meaning don't twist your torso) and knees just slightly bent.  It doesn't take a ton of weight to feel the contraction, so start off easy until you find the right level of intensity.  Also, you might not "max-out" the muscles while you're doing the exercise (a good thing since you use your abs for everything), but the first few times that you do it you should feel sore the next day.