Here's a neat feature we've found while fiddling around with iTunes 11: Its AirPlay popup panel lets you easily select and control the volume of multiple AirPlay devices at once, without having to bother with individual settings windows and its now accessible from the mini player. This way, you can pipe audio to, say, a stereo hooked up to your AirPort Express in one room and an Apple TV in another, controlling the volume on both independently from a window that looks like this:
