I have a ‘spare’ Dell Latitude 7390 (Core i5 9gb ) on this machine. My production machine runs Debian with KDE.
What might be an interesting distro for me to try out on my spare machine?
You haven’t given any info regarding what you want to try.
If you just want to try something different, Arch Linux is an obvious one. A nice learning experience. However, I’d say rolling release is not as recommended on a machine that you’ll be using less than twice a month, since I hear people say you want to update your stuff no less frequently than once a week on a rolling release OS.
So another idea is NixOS. I think it comes with a stable release option? I haven’t tried it, but it’s another option if you want to install something for the learning experience.
If you want something easy to install but different, consider Fedora or OpenSUSE (either version)
For desktop environments, if you want a learning experience for something potentially fun, try a tiling window manager. Sway is one that I’m trying right now (it’s just i3, but with Wayland). Or for something easy but different, any of the big DE will do, like Gnome. I haven’t tried anything other than KDE or Sway.