Very quick and dirty one, this!
I spent five hours (ish) making a quick interactive concept for the Experimental Gameplay Project theme "Night and Day".
I came up with a little driving game where your solar-powered car slowly runs out of juice unless you drive into the light. The objective is to collect coins that increase in points value if the player combos together a longer string of coins in between charges. This creates a bit of a risk/reward thing as the player who plays it safe doesn't score big points.
It's very rough around the edges, but I think the concept works pretty well. I'd love to turn it into something more substantial at some point, perhaps as a proper racer rather than a mindless collecting exercise. It would be pretty cool to sabotage your opponents light sources to get ahead in the race, or maybe each player could be light or dark, and have to turn light sources on and off to gain an advantage.
Not bad for a few hour's work anyway, here it is: SolarCar.