It's not "actual hardware" in the full sense, but the
PiDP-11 is a scale-model of a PDP-11/70 front panel that encloses a Raspberry Pi running a PDP-11 emulator.
The x86 architecture itself has some little-used corners that are fairly interesting. You might try writing an OS for the 286 subset of the architecture (segmentation only, don't use 32-bit segments).
To be honest, though, the "actual hardware" requirement may be fairly limiting for you here if you're not wanting to do something "boring". A lot of older architectures were created while the industry was still figuring things out and before certain modern commonplaces had been standardized upon. They also had to make tough and creative choices about how to deliver the most value in an era when high transistor counts were expensive. Most hardware built recently enough to be readily available is built to get s&%t done in an era where transistor counts in the hundreds of millions to tens of billions are doable.