I've found that the best way to go is by trial and error.
First, it helps to just blue sky your life and think of the craziest, coolest things you would want to do. Next, actually try those things.
Think you might want to work for a chocolate factory? Try it! If it doesn't work out, then it doesn't work out.
If actually interning or working somewhere is not an option, I recommend informational interviews. Do anything you can to actually test out your potential passion.
Before long, you'll find your sweet spot!