The latest entry on the Swift blog highlights additions to the playgrounds within Xcode 6.3 beta 3.

Playgrounds are now represented within Xcode as a bundle with a disclosure triangle that reveals Resources and Sources folders when clicked. These folders contain additional content that is easily accessible from your playground’s main Swift code. To see these folders, choose View > Navigators > Show Project Navigator (or just hit Command-1)

There is an example playground that calculates Mandelbrot set which looks like fun to play with.