Boktai: The Sun Is in Your Hand is certainly one of the most unique games to hit the GBA...or any platform for that matter! The game included a sun sensor on the GBA cart that you used to fight vampires. You'd soak up the sun to charge the game's solar weapons, and then fight off vampires via your solar power. While this is a big step beyond any real-world activity most games had at the time, it turns out creator Hideo Kojima wanted to take things even further.
Shmuplations recently translated a classic 2003 Nintendo Dream interview with Hideo Kojima that's all about Boktai. It turns out Kojima had another sensor he wanted to include, but it was shot down by his dev team. Read up on that tidbit below.
Sometime last year, I think, I approached Shigeru Miyamoto at Nintendo and gave a presentation pitch. In the beginning then, it wasn’t just a sunlight sensor—I actually had wanted to include a sensor that measured how your breath smelled too! I *really* wanted to add that! The enemies are vampires, right? So if you ate something garlic-y and breathed on the mic, they’d all die. (laughs) “Guys, we have to do this!!” But the rest of the staff all hated the idea. ...then I could have had a sunlight version and a breath version. (laughs) A holy water version might be neat too. Sprinkle water on it and they die.