The most fun thing I have tinkered with was a rover with a robotic arm that we designed and made for a competition. I was responsible for the power system, making sure that every motor and servo could get enough power without overloading or damaging any other bit of equipment.
My personal favourite though, and admittedly not quite as flashy, is a Phase Frequency Control Module (PFCM for short). This module is used to monitor and control the frequency of radio waves (frequency meaning how many times it waves per second). This was really interesting for me as it was a challenge to look through drawings of the circuits and work out how it worked, and then applying this to work out how to fix them.