Google is offering "recent" (as far back as April/May potentially) free offers. The current two are;
- $20 Google Play credit
- 90 days (3 month) Google Music Unlimited
Go here to see if you have anything. You'll need to be connected to your Chromecast, but that's pretty obvious.
Already looking at a few games I've had on my wishlist. Great timing too as I've run out of free games that interest me.