Christchurch is the largest city on the South Island, and while it may not be as popular as places like Queenstown, it’s lovely in its own right, boasts lots to do, and, as there’s an international airport, it’s the perfect starting point for your South Island road trip.
If you have time, I recommend heading out to Port Hills and Akaroa! Akaroa is a uniquely French town on Banks Peninsula and one of the cutest little places I’ve stumbled upon on my travels.