There are two important functional programming concepts absent from most Lisps: currying and lazy evaluation, so if a Lisp is used they'd need to be explained separately. Implementing them in Lisp would be a good way of understanding them.

If you want a more typical functional language without type declarations or monads, there's SASL. It's an ancestor of Haskell, but I don't know of any implementation which runs on modern hardware. Interestingly, SASL was once used to write an operating system.