I kind of went down a rabbit hole last year after the Xerox Star emulator was posted here. It was really cool, but super slow, so I felt like emulating another old system. I ended up writing a GUI calculator for RISC OS in ARM assembly, neither of which I had exposure to before. It was a blast. Very interesting system, it's like visiting a country on the other side of the planet.
There's a pretty good emulator, they've got a bundle fully loaded with all kinds of RISC OS tools.