The Plank comes highly recommended because it works your abs without using any movement. To do you stretch out on the floor with feet extended and you are up on your arms and elbows. Clasp your hands, then raise your body so only your toes and arms/elbows are touching the floor, the rest of your body is straight like a plank. Hold this position for 30 to 60 seconds before relaxing. Repeat.
There is also the Side Plank where you lie on your side, leaning on your arm/elbow and with your legs straight down. You will lift your body up so it forms a flat line from shoulders to toes. To help with balance, lift your free arm up to the ceiling as you rise. Hold for thirty to sixty seconds before relaxing back to the ground. Repeat on the other side.
The Suitcase Deadlift uses two weights and flat surface. First place a heavy weight on a flat surface that hits you about mid thigh. Stand next to it so your side is next to the surface. Squat down and grab the weight. Stand up without leaning sideways much like you pick up a suitcase and then place the weight back down on the surface. Repeat on the other side. Make sure you do the same number of reps on each side.
The Glute Bridge is another recommended ab exercise. It is the one where you lie on your back with both legs bent and placed as close to your rear as possible. Left your rear off the floor while pushing with your legs so you are flat from shoulders to the top of your knees. In addition to working the abs, this exercise is also good for the rear.
The Dumbbell Side Lift uses only a heavy weight. Stand upright, with a dumbbell in one hand and place your feet about a hip width apart. Make sure you keep your back straight as you lean sideways with the dumbbell in your hand as far as is comfortable before standing up straight again. Repeat several times, then do it on the other side with the same number of reps.
The Flutter Kick is a good one for abs. Lie on the floor and place your hands under your rear with your legs straight. Lift your legs off the floor about six inches, then kick your legs as if you are swimming. Make sure your back is kept pressed to the floor. Perform the flutter kick for 30 to 60 seconds.
The Bird-Dog exercise is one of my favorites. You kneel so you are on knees and hands on the ground. Make sure you've pulled your abs in and then stretch out your left leg and right arm in opposite directions. Hold a couple of seconds, before bringing down to the ground. Then repeat using the right leg and left arm. Repeat several times.
Each of these exercises can help tone your abs without the need to use crunches. Let me know what you think, I'd love to hear. Have a great day.