Binary digits, a horizontal register, and the cursor that sequenced the computation.
This CocoaWorld taught binary 1 digits to fall into place. When stacked up they would "carry" into one 1 digit in the next place to the left.
I was as first satisfied to perform a correct addition. I used a horizontal line as a floor that would catch falling digits. With two lines, and digits that would wrap around the display vertically, I sequenced the drops to compute the Fibonacci sequence in binary.