The Plank is great, as are most any other abdominal exercises.
Really anything with balance seems to also have at least a fair deal of core involved as well (Superman's are a good example of this.) The Pull-Up and the Push-Up actually take a good deal of core strength as well.
Any kind of training using the Stability Ball or Bosu Ball will tax your core.
As with everything, especially core work, make sure your form is as good as possible and try to engage and feel the muscles your supposed to be hitting during the exercise!
about 1 year ago