Get credit by responding to surveys. Sadly, not available outside USA right now.
Google Opinion Rewards - Android Apps on Google PlayAnswer quick surveys and earn Google Play credits with Google Opinion Rewards, an app created by Google Consumer Surveys.
Getting started is easy. Download the app and answer answer basic questions about yourself. About once a week you'll get a notification on your phone that a short and relevant survey is ready for you. Questions will range from, "Which logo is best?" and "Which promotion is most compelling?" to "When do you plan on traveling next?"
This is currently only available to users located in the US.