I was in nz for my honeymoon 2 yrs back (Oct 2004). NZ is a tourist friendly country, and is relatively easy to drive around other than the few major city (eg Auckland). I got the car rental (Nissan Pulsar for North Island, Corolla in South Island) from Apex,
http://www.apexrentals.co.nz/, which is at NZ39 net per day. While driving, do keep out for scenic sights signboard, stopover for the wonderful scenic view, and short rest. There're also scenic walkway in most of the town, always visit the 'I' (Information) center when you reach the town.
FOr accomodation, most of the them are not prebook (except for 1st day in Auckland), which I find better, as I don't have to spend time looking for the right place. It also make travelling more flexible. For example, some of the town you might decided to stay shorter or longer.
Some of the nights, I've stayed in top 10 holiday park (
www.topparks.co.nz/), which is about NZ$70. After a few stay, I eventaully sign up the top10 membership ($20), which entitle to 10% discount per stay, and is valid for 1 yr and for acommodation in Australia (Top10 holiday park) as well.
Lastly, do try out the quad drive in NZ. I did mine in Kaikoura, as is the cheapest I can find. My Buggy Jump was done in Taupo (NZ$69), and in Taupo, go for the Taupo fall, and craters of the moon (valcono areas) (free scenic walk). Just some of the many interesting things you can find in nz.