Microsoft/Bing Rewards is a fantastic way to get Amazon gift certificates. Just sign up and set Bing as your default search engine, and each search nets you points automatically! You can also snag a bunch of points daily by hitting their site, and at 6500 points (about every two weeks), that's enough for a $5 Amazon GC!
Copper is the BEST game-playing app I've seen -- I scored over $50 in March on IOS (where these apps are rarer). That $5 is just a nice bonus to what you'll really make.