It's YOUR body, you do what you want. How dare someone tell you what to do with YOUR body.
I use a medicine ball (7kg) when doing sit ups.......it's given me great results.
IMO, I would include weight training as well as cardio if you are not already doing weight training. It will not make you bulk up...... it has made me look leaner.
I had a personal trainer tell me to aim for 11% body fat. I did not realise that was unhealthy until someone here (think it was Em) told me that's not healthy.
At the end of the day, if you are happy with you....that's all that matters!