You can also have it play on multiple rooms at the same time. If you open the Alexa app and then go to Settings, there should be a section named "Audio Groups" with a "Multi-Room Music" option. That allows you to create groups of Echo devices (called "Upstairs", "Downstairs", etc.). You can then say "Alexa, play music <group name>" to play on all the devices in that group.
Unfortunately this doesn't work with Spotify, but it does work with Amazon Music.
Also, it doesn't appear to allow a device to be in multiple groups (so you, for example, can't have device in the "Upstairs" and "Everywhere" group).