Hard money

The carriage services are filled with promises of easy money. Difficult ways to make money are harder to quantify apparently, for even articles headed that way don't quite answer the question.

Probably being an optometrist isn't the easiest way to make money nor being a logger the hardest. While most of us work for a living, the ease or difficulty of the work involved won't necessarily serve as a guide to how readily it will make us money.
Fast food workers are unskilled and so the job is easy to learn but, since the pay is correspondingly low, it is far from the easiest way to make money, while anaesthesiologists are specialists whose work is far from easy though the pay is generous.
The Telegraph UK outlines a way to get your investments to pay a ten thousand pound salary

Data scientists and actuaries are charged with making more for the company. Presumably this skill will enable them to monetise their pursuits personally though they lack the capital of the company they advise. Salary is attractive due to the importance of the role and its specialist nature.

The Simple Dollar tells us about those business opportunities that are easy to start as side incomes. It doesn't say these will readily replace a full-time salary nor that they are an easy way to make money per se.

Growing lucerne, fixing taillights, selling budgie accessories.. these are all activities for which we expect remuneration but nothing speaks to the ease or difficulty in making money. Whereas a wage earner can rely on a certain sum being in their account each fortnight, a business owner has no such surety. Some weeks profit is through the roof, others are particularly lean where no one walks through the door. There are businesses that flourish and ones that were never a going concern. It's nice to see eccentric ideas given flight in a shopfront but - unless it's just to keep the proprietor occupied - it's better to have something on offer that people want.

I like Reach Financial Independence with their personal list (and perhaps experience is the only sure measure of what is easy and hard) of both the easiest and hardest ways they've made money.