Google Opinion Rewards
There's almost no limit to the ways you can make money, but most of them require work (that particular balance is known as "the human condition"). However, there are ways to make extra cash without going too far out of your way. Here are a few.
A small disclaimer that should go without saying: nothing in here is going to make you rich or pay your bills. The idea is to make a bit of extra cash while you idle in the checkout line, in a waiting room, or without doing anything at all. With the exception of large capital investments or owning a business, most things won't make you a ton of money for that little effort. However, a few extra bucks here and there is still something, and if you add up everything here you could give your petty cash fund a nice pad.
Get Paid for Answers with Google Opinions and Other Surveys
Recently, Google released a new app to the Play Store called Google Opinion Rewards.
The concept is simple: answer a few questions every once in a while and get some free credit for the Play Store. It won't be much, but the app will send you a notification when you can fill out a new survey and they're typically only a few questions long. Within thirty seconds or so, you could get an extra dollar.
That's a return on your time investment roughly equal to making $120 per hour.