What is the best legal way to invest $500 for maximum return on investment ?

Answer 1

Buy some supplies you need to do some side work if you're a tradesman or a professional. 

Answer 2

There is no 100% way to invest any amount of money even for that same money back so it depends how much risk you are willing to take to try and get a better return. Having said that $500 is basically f*ck all. If that's honestly all you have, just keep it safe because you're probably closer to desperately needing that $500 then being able to build it up. Sorry