It all depends what you like to do and at what pace you want to travel. I lived in NZ for 4 years. Renting a car in NZ is highly recommended. I usually tell people to get a car and a good guidebook and just make it up as they go along. NZ is compact enough that you can do that. There are B&Bs everywhere so finding a pleasant place to stay is seldom a problem. The roads are gernerally great, and if you aren't already, it doesn't take long to get used to driving on the British side of the road. Just practice the mantra "keep left". Be careful not to try and do too much. There's an infinite array of tempting activities and you can get into mischief if try to do too much and rush around in the car. Get lots of advice from friendly locals. Stay away from the logging trucks and be very very careful if you try to pass.

Let us know what type of activities interest and perhaps we can point you in the right direction.