There is always the possibility of making a profit with stocks, especially if the overall economy is good, and the company is well run, and you have lots of skill at analyzing the company (it helps to have “inside knowledge – especially if manage to avoid indictment).
If you want certaintly, but Federal bonds or CDs from insured banks. In both cases you have a more direct issue with ribbis, though if you hold that loaning dollars violates halacha (treating dollars as money rather than as speculative securities since they have no fixed value) you have no business buying bank shares.
If you want a safe investment with a guaranteed return on your investment, give the money to tsadakah and spend the time learning. Next world is above my grade level, but if you are subsequently in need in this world, a good name is better than gold. Asking for help is greatly facilitated if you are known as a Talmud HaHacham who was a Baal Tsadakah, rather than as someone who put all his money into a goyish bank that went belly-up.