I do them once a week, and they span several categories:
- Circuit analysis (building basic circuits out of diodes, resistors, etc. and learning their functions)
- Power electronics
- Communications engineering (for this one, we built a radio that managed to receive two nearby stations)
- Software-simulated circuit analysis
- Software simulation of DSPs
As you can guess, these all relate to electrical engineering.
Fun stuff, but tedious at the same time.