This one seems to be Valentine's Day related, so I'm going to out on a limb here and say there's an expiration on it. For now, any current Chromecast owners (this isn't limited to new buyers or anything like that) can get $6 of Play Store credit by visiting this page and clicking on Continue.
The offer will be the first one on the left and clicking through will take you to the Play Store where the credit will be added to your account.
If you're in need of app recommendations worth checking out and buying with your new found wealth, just check out any of my Wednesday Android Update posts. (Not necessarily the more recent ones though.) I've shared and discussed so many amazing apps and games and what have you, surely you'll find something worth buying that catches your eye.
If you want a few recommends though, in no particular order I've got a few off the top of my head.