'Simple to learn' is always going to be relative - the biggest burden is making the jump from imperative to functional programming, the syntax is of much less importance. In fact, a radically different syntax might make it easier to pick up the new concepts and approach programming from a completely different angle.
Here's a list of languages you might want to try:
- Common Lisp (CL)
- F# (if you're into .NET)
- Clojure (if you're familiar with the Java ecosystem)
Of these, Haskell is probably the most radically different from what you're used to.