[tdfirth]

I always go for a simple lisp interpreter. I think this covers a good range of techniques and gives you a pretty good feel for what is/isn't ergonomic in the language.

I don't generally bother with features like macros etc, I just get enough going to define and evaluate functions in a repl. When you keep the scope small like this it doesn't take very long, and it can be broken up into a few fairly well defined evenings - e.g. lex, parse and eval.

One caveat is that if you haven't written a toy interpreter before then it will take you a bit longer. Once you know a little about the common techniques and patterns it's easy though.

edit: grammar