Define and implement a compiler for your own language. Or implement a standard language or a subset of a standard language. Write the compiler in its own language or a subset that you can hand compile to, say, python. Nothing will give you a better appreciation of the nature of programming languages and their design. Not having to deal with types and storage management is cheating.

If you are simply cutting notches on your bedpost, write a utility or application in Haskell or Erlang. Even more instructive is to write a program in macro assembler.