There's perhaps nothing more retro than going back to the stone age - and you can go back that far with the Stonehenge Watch.
It uses the same technology as the original Stonehenge - using the power of the sun with the 'stones' to tell the time. Once you've mastered how that works, you can check if it works against the 'real' watch built into the other side.
Sold complete with chain, compass and instructions, it retails for $34.9 (around £18).
Find out more at the ThinkGeek website