Roulette table. Gives you almost even odds to double your money. Higher stakes tables if you're walking up with $1600 in your pocket. If you get a drink out of it you can consider that as an extra dividend on top of your returns.

I'm joking. But the question is kinda crazy because it doesn't scale. You're asking for a 100% return in a month. That's 4000% or more annualized return... Assuming you're starting with that initial capital and you want to keep it working. For those returns you need to take massive risks.

Gambling is one way. Options markets is another. (See gambling)

For $100, you could do thrift store arbitrage. There are thrift stores around my neighborhood that sell items for under $1 or $5 that could easily go for $10 to $10 on eBay or Cragislist. You could buy $100 worth of items, and sell them for $200 online. This works for small dollar amounts but is harder to scale because you will spend more and more time shopping and shipping.

I know thrift stores in a particular part of the Bay Area where college students dump clothes with labels like Armani and Versace because they bought too much / can't take it home with them when then move out. I would think women's shoes and handbags would be another area, but would require more specialized knowledge/research.

Lightly-used items can be sold on eBay, while vintage items can be sold on Etsy.

After that, cars and pickups trucks can also be arbitraged by going to car auctions and watching Craigslist. It especially helps if you know a mechanic well or are a mechanic.

The next step is arbitraging small businesses and real estate.

Beyond that, you need to slowly build a real portfolio of defensible business investments.

I was thinking arbitrage too. But from the perspective of buying ads where you find a way to make more money from the traffic than the ads cost.

Street drugs. Only way to parlay low capital into high returns. Not without risk of course.

I wonder how easy that is.

I remember some research that found low level drug dealers didn't make much more than a typical low wage job... sometimes way less. Many had low wage jobs because of that.

That is probably due to that they use some of the drugs themselves instead of selling it. Professional/successful /rich dealers are not also users, i have learned from watching Hollywood movies...

I'd expect something like fashion or microtransaction card games to be easier.

Domain names.

Find investors who would give you 10$ commissiom on hand registered finds, so these investors pay 20$ per name instead of 10$.

You never have to shell out a penny pf your own.

For some domain investors, saving the search is worth the 10$ extra cost.

Back up your quality finds with data.

Rinse and repeat.

A lot of the things that work for $100 do not scale.

OP, just invest in a low cost ETF.