How to respond with a pretty-printed JSON object using play framework?

Play framework has pretty printing support built-in:

import play.api.libs.json.Json
Json.prettyPrint(aJsValue)

So in your case, it would be sufficient to do the following:

def handleGET(path:String) = Action { implicit request =>
  val json = doSomethingThatReturnsAJson(path, request)
  request.getQueryString("pretty") match {
    case Some(_) => Ok(Json.prettyPrint(json)).as(ContentTypes.JSON)
    case None => Ok(json)
  }
}