That being said, plank variations will work all the muscles around your middle (front, side and back) and build your core strength, which is a good thing. There are countless plank variations.
Squats and deadlifts.
I've recently become a convert to the circuit style training of Jillian Michaels (love her Ripped in 30 and Banish Fat/Boost Metabolism DVDs). Her 6pack in 6 weeks is based on the same premise - it's a full body work out with circuits of weight, cardio, and ab exercises, with a strong focus on abs. I haven't tried it myself, but a friend of mine has had excellent results.
Instead of doing exercises that just work the abs, I focus on exercises that work other muscles in addition to abs. Examples being push-ups, deadlifts, squats, and pull-ups. Just make sure you're engaging your abs when you do these lifts. Two birds with one stone.
Also, planks are always a good standard for ab exercises.
The muscle layer is always tight. The "jiggle" is the fat or skin layer covering the muscle. Neither the fat nor skin gets targeted from working a muscle. Fat comes off in the reverse order that you gained it - regardless of whether you work that area or not. So the best strategy is to work the muscles that burn the most calories (legs, upper back, chest).
The abdominal muscles are a thin layer and don't have the capacity to grow much bigger than they are. So working your abs won't do much to fill in the space behind stretched-out skin or subcutaneous fat.
Ab exercises truly have very little effect on the appearance of one's midsection. If you notice your abs improving, it's probably your other exercises, and your diet, that are causing the improvement.
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