If you live in an area where Uber or Lyft operate, why not become a driver? If you're looking to make some short-term cash, you can definitely rake it in by working for one of these popular car-hire apps. As long as your vehicle fits within the specifications of their program, and you have a clean license, you could do this on the side, especially if you're in a crunch for cash.
Specifically, this subreddit has a great, stickied list of sites and services you can use to make a little extra cash. I recommend reading through it in its entirety and thinking about what makes the most sense from a time/reward standpoint—as well as your personal preferences. You might actually enjoy filling out a ton of surveys, say, over dealing with researching and finding new Mechanical Turk projects to complete.
If you are a confident web developer then designing your own themes is an obvious path to follow if you are looking to make money online. ThemeForest sells a wide assortment of developers themes, including themes for WordPress, Shopify, Weebly, and Tumblr, to name a few. Once you have created your theme, get it accepted onto the ThemeForest marketplace and make money each time it sells.
That isn’t to say that you shouldn’t go after the easy money from things like ads, but you shouldn’t have that be your only way to make money. Ads may make you some extra money, but every click on these ads often earns you only pennies. Additionally, the folks who visit your pages don’t like ads, and if your page is slathered with them people won’t keep visiting your page, and they definitely won’t be clicking on those ads.
Blogging is something that requires patience, persistence and discipline. It may mean writing everyday for over a year before you really start to see any money from it. There are exceptions to the rule, but from my dealings with other bloggers, it seems to be pretty common to spend one or even two years building your blog, your brand and your authority, before making any serious amount of money.
Like talking to people and helping walk them through problems? You can bring in extra income as a contract customer support superstar for companies all over the world. Due to the 24/7 nature of online businesses, companies are looking for people in different time zones to help deal with issues that their users are having. Better yet, if you have experience in service or retail you’ll be perfectly suited. Check out indeed or the other remote job boards I listed earlier to find opportunities.