Vacation activities to improve your skills as software engineers?

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

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

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.