People are always looking to have their cars washed and detailed. You could be a mobile car washer and detailer without having a permanent location. Reach out to people you know or make some flyers and put it in your neighbors' mailboxes. If you want to get serious about it, prop up a one-page website or give out business cards. You can make money quickly doing this.
You can sell your ebooks through Amazon's Kindle program or Apple's iTunes Connect, which gives you access to a large majority of the digital-book reading market and the barriers for entry are incredibly low. You don't need to invest lots of money to make this happen, but you do need to invest lots of time, not only in writing the ebooks, but in marketing them as well.
Understand how a niche website works. A niche website focuses on very targeted, specific information. The content must be specific, useful and interesting to your target audience. Successful niche websites get anywhere from 1,000 to 10,000 visitors per month. You build content on a particular keyword, and you make passive income with Google Adsense or through affiliate links.
Regardless if you need to earn some fast cash or we're just talking about making money in the grand scheme of things, there's an important psychology that needs to be mentioned before getting into the strategies. If you study Freud's model of the mind, you'll discover the Psychic Apparatus. It's the three-part construct in your mind that controls all of your behavior.