The volume $V$ of a cube is given by $V = a^3$, where $a$ is the side length. Given that $V=64$ cm³,
\[
a = \sqrt[3]{64} = 4 \text{ cm}.
\]
The diagonal $d$ of a cube is the space diagonal, which can be found using the Pythagorean theorem in three dimensions:
\[
d = a \sqrt{3} = 4 \sqrt{3} \text{ cm}.
\]
This diagonal represents the longest distance between two opposite vertices of the cube.