Definitely skip the rental car and leave the driving to the professionals! In addition to relaxing for the majority of the 13-hour day trip from Queenstown to Milford Sound, I thoroughly enjoyed the narrated tour and fun cultural and nature trivia our driver shared along the way. It was also great to leave the logistics, including boat tour tickets and departure/return points, up to realNZ. Our bus had a glass ceiling which allowed the most spectacular views along the journey to Milford. Be sure to bring lunch and snacks as it was a little unclear what was actually included with the booking (I do believe there was a small cafe aboard the boat however). The weather also varied between warm and sunny to need for a hat and winter coat. Highly recommend!