I was too old to ever get into the handheld game thing, but the latest solar-powered Game Boy does something very interesting: its games whether you’re in the sun, and how much power you’re drawing, and adjusts the game-play accordingly. Some weapons are made stronger, or are recharged, by sunlight; there are enemies that appear or hide depending on the strength of the sunlight; and the enviroment changes with exposure to sunlight (puddles dry up, it gets windy).
It might sound gimmicky, but I suspect they’re onto something here. There’s a lot you could do with context- and environmental-sensitivity in gaming.
[To the tune of Eric Clapton, “Let it Rain,” from the album Crossroads (Disc 2).]