Not sure if you have posted this before but for a couple of years now I have been using the Google survey app. It's very non intrusive and will alert you of you have a survey available. It's part of the app and asks you usually 1-3 questions. All multiple choice and when completed it adds credit to your Google play account to be used on apps or books or movies or music etc.. It varies what you get from 10p to 50p. But it soon adds up. I have found the survey frequency to change alot. It might go from a couple a week to a couple a day. So you can make quite a lot of credits from something that takes 5 seconds to do. Might be worth a share. I get so many free stuff from my Google play account now due to the credits adding up nicely