There are several different types of ab workouts.  You can do them in isolation or can pair them with a total body workout.  The key is to focus on your core.  The internet is filled with different type of ab workouts.

On the days you are tired, and need a rest day, low impact workouts might be what you need.  It is important to listen to your body at these times.  Here is a list of 5 low impact ab exercises.

Crunches 

1. Lie on your back on the floor.  Your back should be pressed into the ground and your knees should be bent.
2. Your hands are across your chest. Contract your abdominals, slowly exhale as you bring your shoulders up. Use your abs to pull yourself up in three counts.
3. Relax your neck.  Take a slow deep breath in as you slowly lower down.
4. Repeat 5-10 times

Scissor Kicks

1. Lie on your back on the floor with your hands just below your tailbone.  The small of your back is on the floor and lift your legs about six inches off the ground.
2. Keeping your knees slightly bent, move your feet in a scissor or flutter motion (bringing one foot down as you bring the other up). Use slow, fluid movements
3. Control your breathing.
4. Repeat 5-10 times

Leg Raises 

1. Lie on your back on the floor with your hands just below your tailbone.  The small of your back is on the floor. Bring your legs straight up towards the ceiling while exhaling in three counts.
2. Inhale and lower your legs down slowly until your feet are just a few inches above the ground. Keep your back pressed into the ground and shoulders back and flat.

3. Bring your legs back up before lowering again.
4. Repeat 5-10 times. 

Reverse Crunches 

1. Lie on your back on the floor. Your knees are tucked in toward your chest in tabletop position.
2. Place your hands behind your head for support as you bring your head and neck up off the floor.
3. Exhale slowly to the count of three as you lift, bringing your chest to your thighs and your nose to your knees. Hold for one second before lowering.
4. Repeat 5-10 times.

Russian Twists

1. Sit on the floor, bend your knees and elevate your feet about 6 inches off the floor.   
2. Hold a 3, 5, or 7pound weight (start light) in your hands.
3. Tighten your core which will help to protect your back.
4. Twist to one side, keeping your legs parallel to the floor.
5. Hold the contraction on one side exhaling and counting to three.  The twist your arms and weight to the other side.   
6. Repeat 5-10 times.

Work up to three sets.  (Repeat set of these 5 exercises 3 times.)

If interested, you can read more about other fitness activities .