Non-Killer Stomach Exercises

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Please I need some help. I am looking for some stomach exercises that I am going to be able to do as if I was a beginner, aside from sit-ups...I am not completely out of shape but I have lost much of my muscle tone from having kids and I would like to get it back :). I have a slipped vertebra in my neck and bad knees so please no exercises that would cause strain to either of those area's [yeah I know I am a pain :)] Anyway any help you can give me would be great...I am looking to tone whole stomach and sides, of course everywhere else needs to be toned to but I figured I would start there. THANKS A BUNCH