No, planks aren't the only core exercise I do, but they are the ones I love.
I do that stability ball roll up move that Cathe does on one of the Intensity series DVDs (roll up, touch ball to feet, roll down, then roll up fast and kick ball with feet). I also do Russian twists, seated. I like crunches on the ball with a 10 pound weight. Also, I do a side plank move with hip lowering and raising, with elbow bent and arm resting on the floor, and with feet stacked on a bosu or soft wobble disk. Three sets of 15 reps each side. It's a killer. This is the move that has my obliques showing. I also like the stability ball core roll ins Cathe does in the I-series before she does the full up pike. I find that all these moves help work the upper body also, maybe this is why I don't have arm jiggle? But also, it's genetics, there isn't a whole lot to my arms which works for me since I hate working upper body but makes it tough when I decide to work it because it takes forever to build any strength. I made it up to a 10 pound dumbbell for tricep kick backs......ONCE!
But, as many have said before, core definition only shows when body fat drops to a certain percentage and this is as true for me as for anyone else. I currently weight 126 pounds. If my weight goes up to 130 and beyond, bye bye core definition! So, nutrition and calories are part of the equation.
I have to tell you though that it has been my experience that running and all the fast power walking that I do (between 5.4-5.8 mph) also really works the core. I propel my body forwards with my whole body, not just the legs and the core is braced the whole time to deal with the speed and keep going while the treadmill belts along. So, you don't necessarily have to do traditional ab work to see definition.
In addition, I do several moves to work the lower body and for stability that include trunk rotation (including walking lunges, no weight but hands at either side of your head and after each step forward there is a rotation over the lead leg to that side, this also makes you unstable as you lunge, so it works the core more than a traditional lunge). There are many ways to get core definition.
Clare