Given your restriction, I'd say stocks. But stocks are inherently risky. And unless you're willing to put in the effort, I'd recommend you stay away.

Further, many companies have cash and some of it will be earning interest. They will also hold bonds of sorts which would also be earn interest. They may raise money in the bond market which would require the company to pay interest. This would severely restrict the amount of companies you can invest in.

Mutual funds typically don't do very well, not when consideration is given for their fees and their performance benchmarked to the S&P.

I would buy commodities like gold and silver except that they're at an all time high. And what your timeframe here is important.