What are some useful computer related technical skills I can learn within a day?

HTML.. You can grok it in a day with dedication. It isn’t hard but will help you understand how the Internet works.

I went from knowing nothing to being good enough to get a job doing internet shit with this book.

I think I bought the book around 1999. I still keep it on my desk.

And I can’t really explain why my finger looks like that.

Memorize 20 keyboard shortcuts.

Youtube’s how-to videos are a great way to learn new skills by watching and then doing. Search for “How to Use a Computer” and start here. Youtube how-do videos are invaluable. I’m a pretty experienced computer user, but hardly a day goes by that I don’t learn something new by Youtubing and Googling.

Excel basics are pretty useful too, depending on what type of work you’re trying to do. Now that I think about it, the answer to this question is really dependent on what you actually want to do with the computer.

The Python programming language is considered the easiest to learn. You can download it for free. There are some very good tutorials. Within a day you should be fairly competent. To master more advanced skills, allow another day or two.

hardware:

learn how to add/replace a hard drive

learn how to install a new version of windows from scratch

learn how to add memory to a computer (different for laptop/desktop)

learn how to replace a broken CD drive. (different for laptop/desktop)

Learn the scripting language of your particular environment. In Linux or Mac this is most likely bash. Windows has a few different scripting languages, of which PowerShell is king. Unfortunately, PowerShell depends heavily on .NET, so if you don’t already know .NET you won’t be able to pick up PowerShell in a day. Visual Basic for Applications is another powerful scripting language. It’s bundled with Microsoft Office, and as its name implies it’s intended to work with applications. I’ve used it for system-level scripting, and I learned the basics in a day. Back in the 1990s I used to write “batch files” for DOS/Windows. I’m not sure if those still work in modern versions of Windows, and I’m fairly sure that the API hooks are lacking. I’d recommend learning VBA for Windows; it’s garbage but it will get you there.
If you’ve ever needed to rename 10,000 files according to a pattern or do any sort of repetitive computing work, then you need to learn how to script! Python is great, and IMO better for scripting than any of the native scripting languages I’ve used, but it’s not available on every system. If you find yourself working on a computer where you are not allowed to install software, then it will behoove you to know something of the local scripting language.
If you already know how to script, then learn regular expressions. Regex is a way of searching for specific patterns. Any programmer that doesn’t know regex is effectively crippled. Regular expressions take only a few hours to learn, but a lifetime to master.