When I left a group I had volunteered with for many years (15+), including many years as the chair, I put it on a meeting agenda as future planning and said it this way:
"I believe that it's not good for the same group of people to lead an organization for too long, because if something unexpected happens, no one else knows how to run the organization. It is better for knowledge to be spread among many people. In addition, people seem to assume that I'm always going to volunteer for things which I think actually prevents new people from getting involved. So, for the good of this organization, which I love, I am leaving this organization at the end of this term. After that, if the new leaders have questions, they can feel free to call me. I wish you all the best."
Then, you have to mean those words. If they call you to ask a question, I hope that of course you would answer the question and share your knowledge (don't volunteer to run an event again, but you can share how it's run - for example, "sure, I can write down the vendors that we used and email that list to you so that you can contact them").
After that, the biggest things that you can do to avoid hard feelings is to not gossip about the things you didn't like about the organization. To anyone, ever. When people ask why you REALLY stepped down, just keep saying a version of the statement above. Those things are all true and they don't throw anyone under the bus.
On a side note with respect to PTOs specifically, to address this kind of issue and prevent burnout, my kids' school PTO recently set up a chair system. People were elected to the position of Secretary-Treasurer, VP, and Pres of the PTO in the first year that they set it up. The second year and for all subsequent years, we elect a new Secretary-Treasurer only. The previous year VP automatically becomes President, the previous year Secretary automatically becomes VP, and the person who was President is required to be off of the board for at least 1 year (previous Pres can be a general member of the PTO, but can't immediately rotate back into the Sec-Treas position). I think it's a great idea. There are always 2 people learning how to run the organization, and you know that when you are President, it will be for 1 year only with a mandated break afterwards - so it helps prevent burnout among the volunteers. If your husband (because he is currently President) wants to consider a long term fix to the problem before he steps down, maybe he could suggest this type of organizational structure to be voted on this spring, with the new 3 officers taking their positions at the start of the fall semester. Good luck.