Idea I've been playing with this week:
"Minimum Viable, Programmable Computer"
Something roughly the size of an iPhone 5/SE that you can directly program in rudimentary assembly with built in keys for opcodes and data
Maybe it can have a display and a few pins to connect to hardware (acting as Yet Another Register you can write to). As a fun exercise, you can maybe set pause the program and increment each "step" in the program counter
I think it would also be a nice learning tool