The Nintendo DS Lite seems like it was made to have solar cells put on it. It has a large amount of surface area on the top and bottom that both face upward when the DS is opened. The top and bottom faces each have exactly the right amount of room for two 60x60mm solar cells side by side (four total). Each one has a maximum power output of 3 volts at 40 ma. The top two are connected in series and so are the bottom two. These two sets are wired in parallel to get a total of 6 volts at 80 ma, perfect for trickle-charging the battery.
Note: A newer version has been posted and is better than this one. Go to: http://www.instructables.com/id/Solar-DS-quotLightquot-Redone-and-Greatly-Impr/
for a better version. If you want to make a solar powered DS the easy way, you can still use this one.
If you have a question, don't hesitate to ask!
By the way, this is my first Instructable!