Hardware Design
Actually designing and making stuff is very rewarding. Layout and design using Solidworks is then exported to CNC. Designing for mass production has its own challenges.
PCB Design
PCB design is a means to an end, it is somewhat addictive, much like Tron's Lightcycle. Design for high speed circuits has to be done very carefully to comply with clock-skew and EMC requirements.
Circuit Design
Circuit design beyond what you'd see on a datasheet is becoming a rare skill. Analogue design is even more rare. I draw the line at microwave frequencies as that's clearly witchcraft.
Computer Programming
I grew up creating assembler demos for ARM 250 chips on the Archimedes, which was a good choice as now there are 70 Billion decendents. I also have used most of the modern languages extensively.
Databases
I avoided databases for a long time. But eventually landed a job where databases are everything. MySQL, MariaDB, Postgres, Informix, SQLite - and multi-site DB's load balancing.
Development Management
IAlways really been the idea's man, but eventually things get to big to fit in one brain. This is where I got in to0 managing a team of developers and larger and larger complex projects.