Have you every tried Pilates? It's great for abdomen strength and flatness. You can also do some of it at home instead of attending a class.
No matter what workout you chose, one thing I would suggest is to do stomach workouts that don't require you to move your spine. For instance, I would avoid sit ups and crunches, they don't work that well, don't show much results and can harm your spine. I would do planks and workouts with resistance bands ( you can get them for very cheap), You will see faster results and you won't hurt yourself in the process.
I know you don't wanna go on a diet, but do consider looking at what you are eating. Sugars and carbs are not good for anyone trying to lose weight around the stomach area. Also, consider lowering your carbs and taking in more protein in return. Don't lower your fat intake! You need fat to burn fat! But healthy fat, of course.
Btw, I say this assuming you are healthy and just wanna drop some pounds in your mid waist areas.