Good reason to give employer for quitting job (without telling them it's because I really dislike them)?

You can try to be vague and say "I have decided to change career fields, and you are going to pursue an opportunity you feel may help you achieve some personal goals. Although you hate to leave your current position you feel it would be better for you in the long run" they may ask specific questions that back you into the corner, just put on your politician hat, and answer their questions without really answering their questions. Let them feel that it is a business decision and not a personal decision.