Offer your services and get paid through Bitcoin. You can also receive payment using other methods, and then you can simply buy Bitcoin and hold onto it. Other than that, there would be risks that you'll lose some of your bitcoin investments, like when lending them. Wherein some borrowers might not pay you.
Perhaps all methods that involve increasing the number of bitcoins carry certain risks. Whatever you try to do, lending, trading, or something else like that, there will always be a risk of losing funds.
Probably one of the most reliable ways, if I may say so, will be to find an additional source of income, and use this income to buy bitcoin. At the very least, you will eliminate the risks that are present in other ways and ensure that your wallet is constantly replenished with bitcoins. Of course, this will require your time and attention, but it won’t work out otherwise, I’m not sure if there is such a way in which you will sit, and bitcoin will increase in your wallet.