Retro and vintage computing
Sun OS and Solraris 2.6
There is a guy running the classic Sun OS 4.1 in Qemu. It is all cozy and soul and the 90s.
https://john-millikin.com/running-sunos-4-in-qemu-sparc
There is however some issues to consider. The best thinkg you potentiall can do on an actual working vintage Sun hardware is to run X11, Emacs and some compilers of decent languges.
The thing is, however, that all the APIs on old Sun systems are grollsy outdated - no modern X server, no nice font rendering, no modern C++ compiler, which means no support for even -std=c++11, which, in turn, means no modern software or libraries.
The best thing you can do instead, is use the most modern hardware while running the most recent versions of classic systems (X11R6) and programs (Emacs, Neovim) with a minimal, bloatfree setup. (i3, rxvt, etc).
One may even opt out for the FreeBSD (or even OpenBSD!) system, which is the closest relative of now long dead Sun operating systems. In short, there is not much to regret, except outdated APIs.