Last weekend I took my sons to Goodwill to drop off some old toys & such. We went into the store to see if there was anything interesting to hack... My oldest son (10) found a Starfighter costume that included a helmet that had fake LEDs across the top. Within a few hours, we'd installed real LEDs and a Modified Electronics "Pico".
It was great to start and finish the project in one afternoon and get some time drilling, soldering and coding with my son.
Full Details & Pics on our blog...http://raisinggeeksandgearheads.blogspot.com/2010/09/september-6th-arduino-project-in-day.html