• Reselling: Buy stable value items (e.g. Samsung SSDs, top selling Canon lenses) on special offer and resell them on eBay/craigslist etc., (requires research and investment, November has the best offers of the year)

• 18+ New customer bonuses: e.g. opening a bank account/crypto market account with an opening bonus (variety of offers may vary depending on your country)

• Affiliate marketing: High entry threshold. For a while I recruited other people in a forum for "refer a customer" offers and shared my bonus with them in return.

• 18+ Sports betting special offers: In addition to new customer offers, some providers also have regular offers (e.g. for odds boosts) with a positive expected value and staking limit. If you are not interested in sports, don't have an affinity for addiction and can deal with probability calculations and betting conditions, then this could be an option. I do this on the side and earn €500-1000 per year.

https://www.reddit.com/r/beermoney/ , maybe you'll find something that you like. Also check if you see something on https://www.fiverr.com/

Try survey sites like Yougov.

If there was a quick and easy way to make money, you would've already heard

Use those survey thingies like Swagbucks

Learn coding and get an internship over the summer that’ll pay you 10-20k

Not online, but lowkey sell candy for like 4x the price it costs.

Selling custom PCs used to be a thing and probably is. Find people that want gaming PCs or anything, agree on a build, build it and sell it to them with a ~%10-%20 mark up. Also all the parts have mail in rebates that are annoying to do but can make you another $100 or so.

Slight risk you might brick a component but it's not hard to do right.

Looks good on any resume too.

Create mobile games for app stores and document the process on youtube. Get money from youtube views, sales, ads, subscriptions etc.