Lisp

Using a metacircular interpreter for lispy languages - one that implements the same language it's written in, allows you to skip the often tedious parsing step by using the host's read to give you a syntax tree, letting you skip to the interesting bits of manipulating and transforming that tree and executing it (eval-apply in sicp terminology). Those bits are foundational stuff for writing an interpreter or compiler for any language, which lots of people find to be interesting projects.