Soon after I completed a study of the x86 architecture and instruction set, I looked for older hardware that would be fun to hack.
The Dreamcast was an absolute classic, no doubt. What's (maybe?) less known is how incredible its memory card was — the Visual Memory Unit. It had the capability to store and play its own games and provide branding for the system. The VMU flash memory contains 128 kilobytes of storage, and working with its customized Sanyo microcontroller and framebuffer was awe-inspiring.
I worked on a game with it and got notably far, having found a toolchain from other hobbyists.