Ever notice how a push-up looks a lot like a plank? Truth is, push-ups are a great way to train your abs, lats, and glutes. Wanna make them an even better core exercise? Add a reach (as described here in the push-up with reach). By creating a longer lever, the reach will require your abdominals to stabilize to an even greater extent so you don't end up falling on your face. Guaranteed, you'll never look at push-ups as strictly a chest exercise ever again.