Well diet isn't always what is the be all and end all. Weight wise you are perfecto. I would just work on those good old tummy crunches. If you can get or have one of those big ballsrest your lower back on it with your legs in a table top position or making a 90 degree to your body fold your arms over your chest or lightly behind ears and crunch baby crunch. its easier on the back, a good workout and it works the butt too.
You can take that ball laying on your back between your feet and raise and lower the ball.
If you like the pool. or have one there are some wonderful tummy exercizes you can do there. Take two of the floating dumbell things lay straight arms out bring in close to body if you can balance and stay afloat and crunch your stomach muscles in so your legs come into your chest and straighten back out again so you are flat on water and repeat, going fast if you can then swtich so right knee comes up toward left shoulder and vise versa. You can do a version of this laying on the floor too but I find it really hard on my neck.
You can lay on the floor raise your legs about 10-45 degrees off floor and leave them there raise your hands (palm down) and pulse holding your legs in this postion.
Even if you are working or walking crunch your abs and straighted your back posture YOu will feel it.
To me you don't sound like you have much of an issue and I am jealous that you can have the weight you have. I have had a baby that was so active in my uterous he tore my stomach muscles so no exercise is going to fix it. I personally don't think you should be worried.