Abdominal crunches are the only way to re-tone those muscles. You also need to continue doing those dreaded Kegel exercises. Everyone hates doing situps and Kegels, but they really do work if you do them correctly.
The Kegel muscle and the abdominals work together - the Kegel supports the internal organs in your abdominal cavity, and the abdominals (along with the muscles in the back) support your torso. So, it's very important to continue working both sets of muscles to keep everything strong and well-supported.
As for crunches, first make sure that you don't have any separation. Lie on your back with knees bent and feet flat on the floor. Put one hand behind each ear with elbows out to the sides. Look up at the ceiling and find a spot slightly behind you to focus on so you keep your chin off your chest. Take a deep breath in, and exhale as you slowly lift your head and shoulder blades off the floor. Inhale as you lower back to the starting position. Start with three sets of 8-10 repetitions, and increase to 12-15 reps each set as you become stronger.
You also need to work your obliques (the ab muscles that run diagonally around your torso, forming your waistline and flattening your tummy like a girdle). To do that, start in the same position as above, but as you exhale, take one shoulder blade diagonally across toward the opposite knee, inhale and return to the starting position, and repeat on the other side.
This should get you started!