Staying still is rarely so effective as however many minutes you can stay in plank. If done properly, it works just about every muscle in your body, and if you absolutely can't with the thought of crunches, it's a killer alternative ab workout.
How to: Plant your hands directly under your shoulders as if you're about to do a push-up. Ground your toes into the floor and squeeze your glutes and thighs. Look about a foot beyond your hands on the floor to neutralise your neck. Slightly tuck your pelvis to keep your booty down and pull your abs in like crazy; you don't want a mini downward dog or a banana back. Stay there for 20-30 seconds, or as long as you can without losing form.
Variations: Spoiler: these are all hard. Play with placing your forearms on the floor, walking your hands further in front of you to build even more strength in your abs, or, if your wrists can handle it, planting your fists on the floor.