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.
If You Want to Do More Than Just the Basics with Your Resignation Letter. You can always write more, and craft a longer resignation letter, but the truth is that it isn't necessary. You can add a portion of the note where you thank your manager or boss for the opportunities and time you have had together, or add that you hope to keep in touch, but in reality, if you really feel this way you probably should make an effort and find an opportunity to say these things in person (rather than getting an email).