I remember being floored by APL few months ago when I learned about co-dnfs and watched the stream by Aaron Hsu [1] with quotes like "This code is perfectly readable, it just isn't in English".
Thing is, when I was thinking about trying it, I never figured out a good toy project. Like, when I wanted to try anything else, most of the time I would know I would be able to cobble together a web-service. Haskell, Erlang, even Prolog.
I understand co-dnfs to be self-hosting, so maybe a toy programming language?