Joining the stock/forex market may seem a bit risky. However, you can start small and continue researching till you gain experience in this area. Once you do, it will be easy to earn from exchanging foreign currencies and/or stocks. Their rates fluctuate depending on supply and demand, and economic and political influences in the world. The aim of any trader is to spot which currency/stock is likely to rise or fall in value against another. The more time you put in, the more you will earn.
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.