There is an app called "AppTrailers" on the Google play store. It's one of those "watch a video, earn points, convert points to a gift card" kind of things. It's the only app I've found that actually works. It's not amazingly fast, but it's reliable.
You can start the videos (I suggest the rapid reviews since they are the shortest), and just let them run. The points earned diminish after a while, but resets again at midnight.
Perk TV is another app by the same company. It's slow, but requires a lot less attention. It runs for longer before asking if you're still watching or finishes.
I use two devices that run app trailers or Perk TV several hours a day, and I get anywhere between $1-$2 a day. I know it doesn't sound like much, but it adds up.