While I can't answer the question of what I would design if I could (though it is a fun challenge I'd like to tackle... eventually), I can say that the same concept-- the idea of a sun jar to collect solar energy for later-- can be made at home, instead of spending 40 bucks on a novelty. There's a tutorial on Readymade (I think !) and definitely on NotMartha.org.

The car gauges in our Prius are an enormous incentive to drive sensibly. Little green cars stack up as you charge the battery, and grey bars go up as you use petrol - it makes you try hard to reach new highs/lows, and the end result is that you end up changing your driving in order to get a better result on the display. We're only human - a little competitiveness and fun goes a long way.

I don't see why all new cars shouldn't have the display - obviously the battery charge isn't possible, but seeing your fuel consumption mount up would give you a visual reminder and something to track your improvement with.

I haven't started yet, but I found the instruction for a DIY Sun Jar here.

http://www.instructables.com/id/Home-made-Sun-Jar/

The components might add up to about $40 alone, but you get the satisfaction of building it yourself!

I made a sun jar. Costs $9.