Given that the language itself is so very mundane, I'd recommend implementing a fun and challenging project of your own choice -- in Java, of course. Preferably one that won't consist of plugging framework A into framework B.

Language-centric tutorials won't get you far in Java because there's not much to discover. Python or Scheme or Haskell elicit a "Wow!" every now and then. Java is all about libraries, tools and ecosystem. It's a language that very much disappears behind its immediate surroundings -- counteract that by extensively using the language.

So go and grab that Play!-thing, and build something reasonably awesome.