What do you mean by "investing in Bitcoin without mining"? Do you want to make more BTC I guess... well its simple, you can buy monthly with extra money you may end up having at the end of the month.
If you want passive gains like investing in hashocean... I think its not going to happen. There's no way to make passive gains.
So either buy BTC, or buy arts and sell for BTC.
But he's worried about trading, he's afraid that if he buys them at a high price, and then it goes down, he will be at a loss. There are many options at Crypto but they are too risky, and mining is a low-risk option, and they have a steady passive income stream.