Great answers by both Karin and Susan so far. Here are some I use:
1. Rectus Abdominal: You can have them do a crunch on a incline bench or stability ball where they would place their hips lower than shoulders; to increase the intensity you would have them sit further horizontal until they can be on the ground or parallel to the floor.
2. Transverse Abdominal: Another idea is to have them simply lay on the floor in a standard crunch position; then contract their abdominal muscles by simply drawing their belly button towards the floor, hold for X number of seconds, and then relax and repeat. They can even do this while seated at periodic times of the day to practice good posture while helping develop core strength.
3. Obliques: Next you can have them standing feet together holding a light resistance band attached to an anchor point perpendicular to their stance. To progress you can increase time, have them kneel, position them further from the anchor point, or use a heavier resistance band.
4. Spinal Erectors: Have your client hold a clipboard or other similar, light object and position them in a Standing Bent Over Row starting position; hips hinged, knees bent with arms outstretched, flat back and retracting their scapula as they hold the position for time. To progress you can add time or have them hold a heavier object. Alternatively you can have them hold the starting position on a 45-degree Back Extension Machine for time, arms outstretched starting with the clipboard and progressing from there.