Build an arcade machine. There are tons of resources online for the cabinet construction and graphics  as well as kits for the controls. It would be very fun and functional and cool
PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...