I'd learn bushcrafting, survival skills, and homesteading. Seems like our earth is headed towards midnight at mach speed, and I've always wanted to learn these things anyway.
I don't think I would retire in the first place. I would go on a long vacation and then prepare for a new startup. Life is not only about making money, It's also about doing your best when you're still young and energetic. "gather your rose while you may....."
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