Action Missions

Action Missions have a lot of rewards. Basically, it's a game where you watch ads, but the ads are only 5 seconds, so it's super easy. You can watch an ad after clearing one stage to get three coins. Actually, you don't even need to clear the stage because you get coins even if you die, so just tapping through the stage is fine. Also, the one where you watch 10 ads for 150 coins is pretty good. The ads are short, and you can collect them once an hour.