Ole Begemann made a Swift Playground that shows off some of the new features coming in Swift 4. Once you download the latest snapshot of Swift, you can run the playground in Xcode 8.3.

I have been following what has been accepted in Swift Evolution for Swift 4, but seeing it and being able to interact with it gives you a much better understanding of what it will be like to work with.

You can download the Playground on GitHub.