Start something from first principles.

Examples:

Instead of using a middleware between a web server and an application, write one (of course it was easier when I did it in 1994.)

Instead of using an IDE that magically recompiles Java and brings you to the errors, write the code that monitors the file system and runs javac, etc (2005).

Instead of using a CMS, write one (did it in Perl in the 90s.)

Instead of using a JS framework, write one (I kind of tried but no time for it.)

You'll find something that's relevant today and for your work.

There is no need to go on for too long because this approach is good for learning but bad for productivity. The problems to solve and the plausible solutions get clear pretty soon. The problem, as always, is the time.