Electronics
I enjoy electronics - coming up with a design, sourcing the parts, designing a PCB, etching the board, soldering it all together, seeing how it works, and finally planning the next version!
I've been playing around with 2 microcontroller platforms - the Create USB interface and the Arduino.
The Arduino is very easy to get going with - there is an excellent tutorial series on their website, ready-made boards are cheap (20 quid - I didn't even bother trying to make my own) and the IDE software works on windows, mac and linux. The ready made boards use the FTDI USB-RS232 chip to provide COM port access to the board from your PC. This makes them easy to set up and easy to write code to interact with your Arduino creation (e.g. using Pyserial, etc). The biggest thing is the active community using the Arduino - lots and lots of example code to base your creation on.
The Create seems to be harder to get going in - the Microchip software is windows only, and the C-compiler is only free to students. And I haven't got used to the C-libraries for it yet (I'm having trouble printing debug output - what does printf mean on a microcontroller?!). Of course the advantage of the PIC's built in USB port is that it can look like any kind of USB device to the host computer - keyboard, mouse, etc, etc, instead of being constrained to look like a COM port like the Arduino.