I once made a circuit which would tun on an LED light when the ambient light fell below a certain threshold using an ATMEL microcontroller. It's actually very easy to do.
The coolest physical thing I tried to make (though it was mostly a failure) was a USB powered fan from scratch. I also tried to make the plastic casing from scratch using resins. I had made a mold for the casing but I had made it out of soft clay (if I remember right) and although the resulting plastic casing did solidify inside the mold and somewhat took the form of the mold, it was too brittle and the two halves of the casing didn't quite line up so they could not interlock neatly around the electronics. I didn't complete the project. I just did it out of curiosity.
The coolest physical thing I tried to make (though it was mostly a failure) was a USB powered fan from scratch. I also tried to make the plastic casing from scratch using resins. I had made a mold for the casing but I had made it out of soft clay (if I remember right) and although the resulting plastic casing did solidify inside the mold and somewhat took the form of the mold, it was too brittle and the two halves of the casing didn't quite line up so they could not interlock neatly around the electronics. I didn't complete the project. I just did it out of curiosity.