(Author of Antilisp, a Lisp for metaprogramming in non-Lisp languages) Not as small a project, but writing a feature-complete implementation of a small language is a very good learning experience that I would recommend to everyone.

There are only so many language implementers, and many opportunities to innovate in language design become obvious when it's your turn to decide what to implement.

For example, I made macros and axioms first-class citizens of the language as special cases of the function type, because using an interpreter allowed me to decide how things should be evaluated at runtime. I accidentally found a trivial way to bootstrap (quote) in my language. I also found a way to make a much more general reader macro system by simply writing a modular value reader that can be extended by adding new elements to a list of parsers. This make non-lisp syntaxes like JSON much easier to implement than with the CL reader macro system where reader macros are bound to some special parameters.

Note for context: In my case, the Lisp interpreter is under 3KLoC, and most of the abstractions are bootstrapped from macros and reflection primitives, or primitive that are shadowed by higher level builtins.