With Sonos, creating a simple two-zone system is an idiot-proof doddle: the cheapest way would be to combine two of the new Play:3 units with the Sonos Bridge router.
Want to go more high end? Hook up a ZP90 Zoneplayer to your existing hi-fi and router, add speakers to the amplified ZP120 in another room, and enjoy.
Upgrade with more Zoneplayers like the Play:5 and control it all with the free iOS or Android apps. The new Sonos Labs platform is in beta and gives users access to even more streaming services. Note: a Sonos multiroom system is now available from as little as $1099.