I've never been on the hiring side, but something I always try to do is be unique. When asked design questions, use something less conventional. Tell them you know it's less conventional, and make a simple justification for it. A simple question I've gotten was designing a deck of cards in C++. Instead of assigning a suit value of 1,2,3, or 4 I chose to use 1,2,4, and 8. While the point of the exercise was to demonstrate OOP, I wanted to spice things up and show I was also comfortable working at the bit-level, using shifts and masks. It probably didn't amount to much in and of itself, but I felt it made me more unique and that much more memorable.