When giving 2 weeks notice, what should I say if my employer asks why?

A polite way to put it is, "I have found a better opportunity." There's no need to go into any additional detail, you literally just need to indicate on paper that you are putting in your two weeks notice. Only say as much as your are comfortable sharing and let them know of any issues you had as an employee during your exit interview. This will help make things better for your soon to be former coworkers.