Fun projects and ideas if you've got a spare IBM VM/370 mainframe, emulated or otherwise: coding in BASIC and Fortran, playing games, transferring files, running a text editor, accessing the file system, and more.
https://rbanffy.github.io/fun-with-big-computers/fun-with-vm370
P.S.
While I don't possess incontrovertible evidence that VM/370 (VM/SP, ..., z/VM) is the closest to unbreakable operating system there is, it definitely makes that impression.
@amoroso