Meeting up with Mike Lee at today's OLPC Learning Club DC meeting, I remembered his amazing contribution to XO hardware hacks from the last meeting: a XO camera two mirror periscope.
While Mike's rig was very alpha, it served its purpose. You can now use the XO laptop to take photographs of images behind the screen.
Better yet, Mike has a call to action for crafty XO laptop users - develop a XO camera periscope pattern for children to cut out of cardboard. They can then glue on thin plastic mirror sheets and everyone can have their own periscope.