$ 20,000 is a lot of money but $ 40,000 would be nicer, so let's explore what you could invest that money in and consider possible Scenarios that can double it in a month or less. This type of money is quite a decent amount of capital, however, it is not enough to buy real estate without taking out a loan. To double your money at any level is quite a feat, but as you will see, not impossible.
First, a word about risk. All investors look at the downside or the risk profile of a potential investment before they even think to consider investing, no matter what the promised returns are. Risk is an important consideration and it would do us good to define the ideal risk free investment so we can picture better what to look for.
First, when you hand over your money to another, that is the underlying cause and underpinning issue of risk. You have to trust and you have to hope that your money will be returned with the promised interest yield. When you hand over your money in most situations all you get is the evidence that your money is with them. You get no tangible collateral in exchange for that money.
Consider a house on the other hand. A house, or any tangible item for that matter that has an existing liquid market, is a large piece of collateral and you have direct control over that asset in exchange for the money you paid. This means you can control the value to a degree and this is an important point.
To double $ 20000 in under a month, you could look at a land investment. A paper re-hab, unlike a normal re-hab where you roll your sleeves up and paint a house, a paper re-hab is one where you fill in a few forms and immediately increase the value of land. For instance you could purchase a lot that is close to an industrial area and re-zone it industrial from residential. Providing there is a distinct shortage of industrial land in the area, you have just increased the value of that land tremendously. Or you could put in the paper work to split the block to subdivide. This would mean you have 2 lots almost worth the same as the original. This type of value manipulation is critical to your risk profile and the ultimate level of returns.