I was going to say Elixir, thanks for mentioning it.
I'll also add that Haskell, Clojure, and other functional languages will help you think in an entirely new way to program. Python or Ruby are also less mind-expanding, but was a nice change of pace for me at least from my C# day job.
Learning window functions and more advanced bits of SQL that an ORM doesn't give you for free can also be a fun time, along with getting deeper into what indexes can really do.
I'll also add that Haskell, Clojure, and other functional languages will help you think in an entirely new way to program. Python or Ruby are also less mind-expanding, but was a nice change of pace for me at least from my C# day job.
Learning window functions and more advanced bits of SQL that an ORM doesn't give you for free can also be a fun time, along with getting deeper into what indexes can really do.