What languages, niche or otherwise, do you already know? In the absence of context, I would suggest Forth or Prolog, as they are sufficiently different from an

What languages, niche or otherwise, do you already know? In the absence of context, I would suggest Forth or Prolog, as they are sufficiently different from any mainstream language. Of course, Lisps and Schemes are always fun, and I think Racket sets itself apart from those with its language making capabilities. Logo is also very fun, but it is essentially a Lisp/Scheme. There are a lot of neat older books using Logo in various domains. Pharo is also a fun choice.