Three questions:
1) Are you in a field that lends itself to consulting work or other kinds of self-employment? As in, do companies like yours hire outsiders to evaluate them, suggest improvements, or do short-term work?
2) If you left your job (on good terms), would it be feasible to be re-hired in a year or two if retirement really doesn't suit you?
3) Is there anything else you could see yourself doing that would feel productive and structured, in a perfect world? Volunteering, teaching, crafting, mentoring, writing?