Married couple, Jacob and Susan D'Aniello, began their own pooper scooper business. Their small start from working out of their home turned into a multi-million dollar franchise called DoodyCalls. In the beginning, it was just them, a bag, a little shovel... They started the business when they were dating and basically fell in love over canine feces, and while the business became complicated, in the beginning, it only involved a little time, some exercise and then collecting the fee a short time later.
If you're skilled with computers, you could set out a shingle and become a part-time computer repair-person. If you know how to cook, you could start to run a catering business out of your home (but check with your state; preparing food for the public out of your own kitchen can sometimes run afoul of the law).
If you want to get some quick cash, certainly there are limitations to what sort of business you can begin. If you need a lot of inventory, you're going to have to spend money to make your money. But if you're going to do something that takes your skill, talent or knowledge, you can bring in cash quite quickly. No matter what, though, provided you enjoy what you're doing, if you start a business, whether you're successful or not, it's always rewarding.