maybe something that will build the core muscles more gradually like Pilates? It's good because there are several levels of each exercise so you do whatever you feel you can. The nature of Pilates means that everything you do works your core even when you don't think it is. It did it for several years and really enjoyed it, plus my core and arms were quite strong.
However, from a general POV I prefer Yoga these days, as it has more holistic benefits aside from the physical workout, and actually helps you understand your body's strengths and weaknesses and helps you be more accepting of what you have. So although it won't necessarily eliminate the stomach fat (which is the hardest area of the body to tone anyway!), it might just make you feel better about it!