Well how 'bout this:

You could take the calc and make a custom case ( you could make it into a mini-laptop ).  On the fold-down door, you could put a solar cell on the back that would be connected to something that would charge the batteries when it is running (or off), and could also save the batteries more by powering the calc, too, and then if its power alone isn't enough, it could combine it with only as much that would be needed to maintain six volts.  When I have time I should draw a picture of this (I forgot - I got a scanner now! )

It could also have one of those bright energysaving lightpanels on either side of the screen inside for night-time illumination. (and you could change the color too )