However, I'm happy to help out however I can in the next two weeks." If you haven't landed a new gig yet or your new start date is negotiable, know your limits. If you're willing to stay an additional three weeks, for example, don't let your manager talk you into staying three more months.
On the flip side, if you loved your job, your coworkers, your boss, the cappuccino machine, everything about the place, don't write page upon page waxing rhapsodic about all that you're giving up by leaving. A brief, heartfelt compliment to the company, or a thank you, is fine.