Git patches blew my mind when I had a very similar experience. Only thing is, at the time I had around 15 years of experience, but the person with the patch had about 25, so I felt like a newbie.
If I were to guess as to the most mind-blowing old-school thing, I would suggest building a website with a text editor, sFTP, and PHP. There's so much noise around DevOps, containers, PaaS, serverless, etc. that most developers never experience the joy of just adding a life of code, transferring it to a remote server, and seeing their website work with no compile, no source control, no deploy scripts, no CI, and no multiple environments.
This really is the first thing that came to mind for me, with the addition of localhost development with LAMP/PHP. I still just spin up Apache on an unused port on my Linux box for each project I'm working on, and edit my files locally. I sometimes hear "you can do that?"
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