Stay the fuck away from the U.S markets, invest in countries like Canada and so on that are keeping their distance from the dollar I would always say that cryptocurrencies, gold and silver are your best bet but if you are comfortable investing then go for it, what I would do is wait for a crash because even the countries dumping the dollar will be affected and it will be easy to buy up otherwise expensive international companies.
A guaranteed ROI in a few months is hard to find but you could buy oil, sugar, toilet papers and such in bulk and save up to 30%. It is a good and easy deal that will guarantee you a nice profit with an investment of 500$.
You can keep the money on your checking account to avoid overdraft fees if you usually get those.
Try to think of an investment of 500$ that will save you some money on gas, electricity, transport or such.