I would suggest anything that completely tangential to software engineering. In effect, get a hobby.

Or just relax. People generally don't relax enough.

Thank you. I took the vacation for self studying and not for relaxation.

Depends on your career. Software dev is rather enigmatic. I breath air, what should I do to improve being an air breather.

Just speaking for my sphere (BE rails/elixir with some FE).

Take a course on SQL. Dig deep. Chances are good it will make your apps at work faster.

For rails, learn about middle ware. We don't use it nearly as often as it would benefit us.

For elixir, I'd study plugs more. Or create a pile of service modules and play with them.

Another option. And perhaps more fun. Munchkin apps days.

Often we get in the habit of taking our time and living with our world of distraction. Every once in a while I'll take a day and an idea and pump some techno. I build a whole micro app in that day. I try to push myself as hard as I can to make something I could launch. These days are super fun, and I feel some of the joy I had when I started coding 20+ years ago.

Having life experiences like travels really makes you a better software engineer. But if you just want to watch videos and code, take The Primeagen channel on Twitch and replicate any Rust project he's started.

You could work your way through some of the "Linux Programming Interface" by Kerrisk.

Even if you only use high level languages, the LPI provides insight on what's happening under the hood. It also gives important context for when you do performance analysis and program optimization. Each chapter has a bunch of exercises at the end.