Hi steve_n and welcome to LQ.
The
Linux - Embedded & Single Board Computer forum is probably one you'll enjoy.
Also recommend you experiment with some of the various Linux based System On Modules (SOM), Beagle, Hummingboard, Rasberry Pi, are a few. The Pi is probably the most prolific and there are tons of projects out there for various technical projects involving things like GPIO, I2C, SPI, plus you can also attach displays, write your own custom bootloader and kernel (not that you can't on any computer).
Enjoy!