If you have just a few minutes, you could spend it filling out online surveys. There are multiple sites that will pay you to do so and it’s very easy. All you have to do is register, and start filling answering questions. Typically these are online market research surveys for big brands. Some companies even pay you just for installing an app or playing a game!
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.