Depends on what you mean by "extra".
Ideally you should identify your financial goals first, assess your risk tolerance and capability second, then establish an investment plan. That would then tell you what to do with that "extra" cash.
Lots of choices for investments. Stocks, bonds, mutual funds, collectibles, real estate, commodities, and furthering your education and skills are the main categories I can think of off the top of my head.
Also lots of choices for avenues, but it sorta depends on the kind of investment. Brokerage accounts, direct investments with fund companies.
If it's truly extra I'd suggest finding a charitable cause that is near and dear to you and contributing to that.
Read. Learn. Decide.
Good luck.