Work.

That would have been my 1-word answer, but the site tells me it's too short.

Yes, you could invest your money in the hopes of getting a higher return; and it might work. But any investment entails the risk of loss. And that risk is highest early on when a person is less experienced and doesn't know where to invest.

I wouldn't want you to lose your hard-earned money or have a bad experience at 16 that sours you on taking risks. Realistically, the way to add to your stash without any chance of losing it ... involves finding paid work.

Local jobs for 16-year-olds won't pay a lot. But online you're ageless. If you learn a skill such as web design, video editing, or programming, then you could earn more per hour and freelance as much as you like.