Metal Gear Solid | Game Boy Color
Forgot to mention this, but there's another game I'd like to recommend: Boktai. It's a GBA game from Konami that uses a little solar cell in the cartridge. Having the sun shine on the cartridge will alter certain things about the game world, powering machinery, recharging your energy if you're outside, and slowly building energy in solar stations you can exchange for in-game money. Also, in the second game there's a one eyed grandpa that forges weapons for you.