Giving a reason to leave a job to your manager

I'm planning to leave my current job; I like the job but I dislike the management, but obviously I can't actually say the latter to the manager (burning bridges and all). I'm leaving a bit early as well instead of the normal 2-weeks notice period so she might want to know my reasons for leaving early as well.

I want to be as civil as possible but at the same time, I really don't want to give too much information on why I'm leaving; she has the habit of asking too many personal questions. And, she likes to spread the news to other employees through gossips and such.