If you live in a tourist locale, consider doing walking tours on Viator, which is one of the largest platforms for tour operators in the world. You'll really need to work hard to get a good reputation. To find clients quickly offer free tours and ask for tips at the end.
This is leveraging the principle of reciprocity with the power of free. People love to get things for free, but they feel obligated at the end to give something in return. That's why supermarkets are always giving away food. They know if you like it, you'll buy it.
2-Wash or detail cars.
People are always looking to have their cars washed and detailed. You could be a mobile car washer and detailer with 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.
3-Get a small micro-loan on Fundera.
You can get a small business loan (very small) on Fundera, or Prosper if you're looking for a very small personal loan. Depending on your credit and your financial and employment situations, you can secure up to a few thousand dollars
4-Medical studies participant.
If you don't mind throwing caution to the wind, consider becoming a participant in medical studies or drug trials. Companies pay a significant amount of money to participants.
You'll need to find the companies conducting medial studies and expect side effects. If you can live with that, then you can definitely make some money fast without too much effort.
5-Deliver for PostMates.
You can earn some cash when you're in dire straits by using PostMates, and you don't even need a car, just a bike for deliveries in busier cities and downtown centers. I've used it often to get food from specialty places that don't deliver themselves, but it can be used for many things.
PostMates states on their website that you could earn up to $25/hour doing deliveries. You could also earn some tips in the process so it's a great option for people that are looking to do something in their spare time, similar to the big ride-sharing apps that are hogging up app stores everywhere.
6-Drive for Uber or OLA.
Companies like Uber and Lyft offer a great opportunity to make some quick cash. You'll need a clean driving record, a fairly new car and the authorization to work wherever it is that you live. If you have all of those things, you can work when it's feasible for you, whether that's in the middle of the day during rush hour, or in the wee hours of the night on a weekend. The choice is yours.
7-Sell old books on Amazon.
Amazon makes it fairly easy to list and sell old books on its marketplace. You can make more than just a few bucks If you have pricey text books from college. Be sure the books are in good condition. You'll get negative reviews if you attempt to sell books that are falling apart. Remember, add value where you can, no matter how small it might be and no matter how few people might readily notice it.
8-Sell, or resell, used electronics on Craigslist.
The market for used electronics is vast. You can use Craigslist to part ways with your old iPhone or your MacBook. You can grab a premium for these types of electronics. Some people even flip items by buying them and reselling them.
9-Do tasks with TaskRabbit.
TaskRabbit lets you tap into an existing marketplace of people looking for help locally with chores. The tasks aren't always small. This platform (along with others) offers bigger tasks, such as home renovations, as well.
Amazon is getting into the task business with Amazon Home Services. The services listed there range from very small repairs to larger and more involved tasks that would take considerably more effort.
10-Flip real estate contracts.
If you're thinking this is cost prohibitive, it's not. You can flip real estate contracts without ever taking possession of the property. This isn't a fixer-upper endeavor, just securing a contract and selling it to an interested party. That's it.
You can do this with just a few hundred dollars in most cases to open up escrow. There are companies dedicated to teaching how to do this like REWW. It's good for making a little bit of money, but is also potentially something more long term as well.