I would recommend Slate
http://slate.tunes.org/

It features:

 + Smalltalk like syntax (so easy to adapt for Ruby guy)
 + Multipledispatch
 + Prototype base OO.

So In learning Slate you will get two new Idea (Multipledispatch and
Prototype base programming)

I think the idea is nice to learn.

In functional way, may be you would like to try Haskell.

BTW, I'm learning Lisp for myself.

So may be you will be insterested.
A couple things I like in CommonLisp over Ruby is MultiDispatch and
Condition System.