A quick and simple workout routine that can help you strengthen and tone your abdominal muscles, leading to a flatter tummy:

1. Plank Hold (1 minute) - Start in a push-up position with your arms straight and hands shoulder-width apart. Keep your body straight and engage your core muscles. Hold this position for 1 minute, or as long as you can.

2. Russian Twists (1 minute) - Sit on the floor with your knees bent and feet flat on the ground. Lean back slightly and lift your feet off the ground, balancing on your sit bones. Hold a weight or a water bottle with both hands and twist your torso to the right, bringing the weight beside your right hip. Return to the center and then twist to the left, bringing the weight beside your left hip. Repeat for 1 minute.

3. Bicycle Crunches (1 minute) - Lie on your back with your hands behind your head and your legs in a tabletop position. Lift your head, neck, and shoulders off the ground and bring your left elbow towards your right knee while straightening your left leg. Repeat on the other side, bringing your right elbow towards your left knee while straightening your right leg. Continue alternating sides for 1 minute.

4. Reverse Crunches (1 minute) - Lie on your back with your hands by your sides and your legs lifted straight up towards the ceiling. Use your abs to lift your hips off the ground and bring your knees towards your chest. Slowly lower your legs back to the starting position. Repeat for 1 minute.

5. Mountain Climbers (1 minute) - Start in a push-up position with your hands shoulder-width apart. Bring your right knee towards your chest, then quickly switch and bring your left knee towards your chest. Continue alternating as quickly as you can for 1 minute.

6. Leg Raises (1 minute) - Lie on your back with your hands by your sides and your legs straight up towards the ceiling. Use your abs to lift your hips off the ground and slowly lower your legs towards the floor, keeping them straight. Stop just before they touch the ground and then raise them back up towards the ceiling. Repeat for 1 minute.

7. Side Plank (30 seconds each side) - Start in a plank position and then rotate your body to the left, balancing on your left arm and the outside edge of your left foot. Stack your right foot on top of your left foot and lift your right arm towards the ceiling. Hold for 30 seconds and then switch sides.

Perform each exercise for the prescribed amount of time and then move on to the next one without resting in between. Repeat the entire circuit 2-3 times for a complete workout. Remember to focus on proper form and engage your core muscles throughout the entire workout.