The Oberon programming language is a nicer-than-average descendant of Pascal and Modula-2. The compiler was reasonably fast even in the 90s.
The UI was strange enough to feel really interesting. As the article mentions, you could type a command anywhere and click it. And the older version I used had tiling windows, although this seems to have changed.
But one especially neat feature is that the system was based around a rich-text format that you could extend! I wrote a little clock widget that I could instantiate anywhere, inside any rich text, and it would give me a live time readout.
In practice, Oberon already felt like a toy system when I used it, because it was missing tons of useful software. But as toy systems went, it was rich and original and well-worth a couple of weekends of exploring and coding.
If you enjoy the idea of operating systems that explored a different path, this is one of the more interesting ones. If you're fascinated by systems like the Lisp Machine, or SmallTalk, or OLPC, where the entire system is meant to be easily understandable and hackable, take a look.