If you download app you will be sent short 2 second surveys about shops you have visited or other random things like YouTube videos you might have watched. Completing the surveys is really easy and it gives you roughly 15-25p in a credit for the Play Store.
I recently got to my max free capacity for Google Photos back up but could get 100gb extra for £1.89 per month. These surveys should easily more than cover the cost as I’ve built up an average of £25 a year in credit from them.
Hope this helps someone! You can obviously also spend the credits on music or apps or whatever you want (except doesn’t work for Kindle books unfortunately).