I let my boss know in person (actually, via phone call as I was in a different office and state) and gave several weeks notice, as my job was somewhat specialized and they needed time to decide what to do with my headcount. I didn't need to do this much notice, but I'd been there a long time and had a good relationship with my boss and other business partners. As it was the partial truth, I let them know my resignation was for family reasons and didn't go into a ton of detail. I followed this up with a more formal written communication.
BTW, my company also had retiree health insurance, but it was massively high in premium for under-65 employees, so I had to look elsewhere for my benefits. If your retiree benefits were reasonable, it is surprising that no one looked into them.