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.
For example, my husband and I use one credit card on everything and we pay our statement off in full each month. We take advantage of the cash back rewards and have accumulated over $600 just by spending money on our essentials like clothes, bills, food, etc in the past 8 months. I checked to see if our credit card company had a referral program and they did! $100 for every person that signed up using our referral link.
Set up a roadside stand. Depending on where you live, you could profit handsomely by setting up a roadside stand. If you live near a resort area, for example, you could buy cases of bottled water, put them on ice, and sell them to passers by for twice what you paid. Selling fruit and produce you grow yourself is also a smart idea in highly-traveled areas.
Write and publish an eBook. You don’t need a traditional publisher and financial backing to publish your own book anymore. In fact, Amazon.com makes it possible for you to publish your own eBook and sell it independently – with no financial investment on your part. And with Kindle eBook publishing, your book will appear on Kindle stores worldwide within 24-48 hours. Just be prepared to market it yourself on social media, your blog, or elsewhere if you expect to generate sales.