You could develop software to largely automate the municipal bureaucracy of a village in your nation. It takes a surprising amount of paperwork, and manual labor, to perform even the minimal amount of compliance work to maintain a village.

You could work with leaders in each tax levying jurisdiction to agree to publish tax data in a standard online format.

You could develop a backyard automated chicken coop, Creative Commons the plans and see what improvements everyone else comes up with.

You could develop a vanadium redox battery-powered lawn light, and save landfills from the garbage disposable lights the big box stores inflict upon us today.

There is simply an endless sea of opportunities to imbue life around you with increased cognitive density.

My head fills with an idea or two every day (usually about improving something I run across in daily life), that acts like an earworm, which I have to write down to "purge out of my mind". It got much "worse" with the advent of search engines; when I was "stuck" with school/university libraries, I would often run into dead ends researching ideas, and could quell the earworms with the thought that I gave an honest effort to run down a thread of an idea. I thought everyone thought like this, but was just better at focusing upon the task at hand and banishing these idle thoughts. I much prefer the situation today with search engines: I'm much faster at running down enough of an idea and putting it into writing to purge it out than before.

"Man's Search for Meaning" by Frankl graphically illustrates the importance of having a sense of purpose, which I think a lot of people lack when they retire to live the dream of doing nothing.

I think the key is to be able to choose not to have to work for some asshole boss, or do work you don't generally enjoy.

I've become very disillusioned by the dream of insane wealth and the ability to retire early. Instead, I want to work on something I feel will make a difference and that interests me for the foreseeable future.