Google opinion will give free money when you take surveys. Also, there are sometimes promos for free Google play credit tied to certain hardware. I got $50 Google Play credit for my LG G Watch. I also got $50 Google Play credit for my Samsung Galaxy Note 3. Keep in mind that these promotions are not currently active, so you will have to keep your eyes peeled. I have also seen Google play credit for chromecast
Sent from my Verizon Samsung Galaxy Note 4