I also think that doing some AB exercises will help as you lose fat too. These will help tone the abdomen as well. There are many that you can try that won't cause injury. Doing tummy tucks or squeezes or holds are being recommended lots of late -- esp these two kinds ...
The easiest one (which I do every day): You lay on our back on the floor, couch, or bed -- and squeeze your tummy muscles; and hold them for as long as you can. You can start with 10 seconds and up that to 20/30/60 and more as you go. I do some for less time but more often; and some I hold longer but do only 10 reps.
The other ones are called PLANKS, and are done on the floor where you do the beginning of a push-up; lift up, hold and squeeze your tummy muscles as long as you can; and then drop down to the floor & rest for a few seconds then repeat about 10 times. You will likely have to work up to that many reps though.
Other experts also recommend leg and knee lifts for the abs and lower tummy as well. Riding a bike also helps the lower tummy as my DH can attest to. He is slim but gained all his extra fat in his lower belly; after a summer of riding his bike, his abdomen is lean and toned. I noticed this when he was doing something outdoors without his shirt on yesterday.
A trainer once taught me a special kind of situp for women that is more gentle on the neck & back: they are called lady's situps ...
LADY'S SIT-UPS:: start as a normal sit-up, but only bring your back up about 6-12" off the floor; lower, rest and repeat. Start with 10 reps, working up to 25; then change your arm positions. The traditional style puts hands behind your head. Switch to -- 1) Indian Chief: arms crossed in front of you. 2) Prone: arms straight in front of you. 3) Chest: arms crossed tight to your chest. What's nice about these is that they are easier to do, and (for those with special challenges) you can do them sitting on a chair too.
Lifting weights or heavy objects can help strengthen the stomach area as well as others have mentioned. I do toning exercises every day; I do think they help too! I have the same problem and have lost lots of inches in areas that I do them, so I say -- DO THEM ALL, esp while you are still losing fat/weight!