I would recommend looking at Scala, F#/SML/Ocaml and Haskell -- but Scala in particular:
The reason for this is Scala shows that a "fun" statically/strongly-typed high-level (in a real sense) 'OO' language can exist. There are many useful programming constructs that are not possible in Java due to design and implementation limitations. (Scala is by no means a perfect language.)
Even learning just C#3/4 (which generally has a less powerful type system than Scala, although not all areas overlap) will open eyes with constructs that are not easy to do in Java -- simply passing around "functions" as objects or being able to uniformly and easily run queries and manipulations over lists are something out of reach with Java. You can fake closures/lambdas in Java (look at anon-inner classes or the Functional Java library), but it is hardly pretty: Scala and C# add language support for these constructs so they are "natural".
I won't speak ill of Ruby or Python (or many other languages, excluding PHP ;-) -- but one thing that they lack is static typing. Some people find this a blessing, but I generally find it a curse (good static type systems can prove the absence of a number of type problems) -- the problem is that, if Java/C/C++ is/are the only statically typed language(s) you have dealt with, you will think that static typing has to be painful (and implies "too much type annotation" and "too brittle/closed types") -- this is simply not true in more advanced statically typed languages with type inference, view-bounds, typeclasses, etc.
Learning Haskell will really open your eyes up to a different approach -- but it really is a different beast and requires forgetting how language X works. You can write "Java in Scala", even if not advisable/idiomatic, but you really can't write "Java in Haskell".