A new automatic playlist has been added to the Google Play music application called the Queue. This list lets you queue up any combination of playlists, albums, songs, and Instant Mixes without having to create a brand new special playlist for just one listening session. If you know what you want to hear next, you can add it to the Queue and keep the music playing. Before this feature was implemented, it would have been necessary to stop what you are doing to start up the new songs. This will also make playlists more useful, as you won't have create dumb mixes like Three Songs While I Brush My Teeth, you can just queue up the three tracks and be done with it.
To add something to the queue, just click the arrow next to the entity (song, album, playlist) and select Add to queue. It's as simple as that, and you can add multiple tracks at the same time just as easy. The Queue also provides the option of saving its contents to a new playlist, adding yet another way to edit your Instant Mixes.
It also educates us on how Google Play creates its shuffled mixes. If you have a large amount of songs marked as Thumbs Up (1,952 here) or click Shuffle all on the Home page, by clicking on the Queue you will see that Play actually selects a sample of only 800 of those songs. This is probably well over 48 hours worth of music, ensuring that you never hear the same track twice (unless you have uploaded multiple versions of the same song).
There isn't any official documentation on using the Queue yet, so the best way to get the most out this feature is just to give it a whirl.