Most roads, especially in the south, are 2-lanes and very curvy. While the speed limit is usually 100kph, we often found ourselves averaging closer to 70kph. With so many amazing things to see and knowing that the islands aren't that big, it's easy to get quite ambtious! While it would be sad to remove any adventures from our trip, I could have happily traded some of the 3-5 hour drives for more time in Wanaka, Takaka, or Fjordlands. So, before you start planning, take a deep breath and admit that you can't do it all.
Our go-to guide were the NZ Frenzy books by Scott Cook, one for the North Island and one for the South Island. It's not your normal guidebook as it focusses mostly on finding fun places to explore that you would otherwise overlook. Among others, it lead us to dolphins/penguins, Clifton cave, a beautiful forest walk, a fun wire bridge, and a very hot stream flowing into a cold river. To get a full picture, we'd suggest pairing it with a more typical guidebook.
Much to Jonas' chagrin, my dream car is a camper van. Our trip to NZ let me live out this fantasy. More practically, the van gave us flexibility. We didn't have to plan ahead for where to stay and we could make dinner/breakfast easily. The fact that it rained lots made the van extra nice. We were happy with Backpacker Sleepervans, but there are many other options. Be sure to book the van early as there is a lot of demand during high season, book ferry tickets between the north/south island early, and read up 'Freedom Camping' in NZ.
Alternatively, "backpackers" or hostels seem to be quite popular as well as tent camping or AirBnB. If you're planning to spend a lot of time with friends or on back-country trips, then the extra cost of the van might not be worth it.
Walking is a great part of New Zealand culture, and the DOC (Department of Conservation) has 9 designated Great Walks around the country. Milford is the most well known. However, our Kiwi friends recommend Heaphy and Routeburn, some hikers we met on trail recommended Kepler, and some other friends from SF loved their canoe trip on Wanganui River.
We loved Heaphy, the walk most known for it's ecosystems. The huts and trail were incredibly well maintained. We also did Abel Tasman as a kayak, but were unlucky with winds, currents, and weather making it a bit more epic than ideal. One piece of advice: don't forget to print or cache your hut reservations!
New Zealand boasts thousands of glaciers, but Fox Glaciers and Franz Josef Glacier are particularly well known for their size and accessibility. We had hoped to do all-day ice climbing at Fox Glacier, but due to bad weather coming in we switched to just a morning glacier walk. It was still quite impressive. The guides were incredibly professional and the operation very well run. Sadly, these glaciers are retreating quite quickly so don't delay.
While we tend to be do-it-yourself-ers, for some adventures you need a guide, or pilot (literally). We generally had great experiences with the companies that offered guided tours. We wholeheartedly recommend Deep Canyon for Canyonering in Wanaka, Golden Bay Air for the flight transport for Heaphy Track, and Fox Guides for exploring the Fox Glacier. In fact, our only unprofessional experience on the trip was with Real Journey's Milford Sound Kayaking. Based on our experience, we would recommend using Rosco's instead.
Our list of "things we want to do next time" is even longer than our list of "awesome stuff we did!" It includes: Kepler Track, hanging out in Takaka at Hang Dog for nearby caving, mountain biking, climbing, and swimming, visiting the geo-thermally active area of Rotorua which is known for its mountain biking, checking out the 90-mile beach in the far northand ...
likes scrambling and swimming
likes long walks and waterfalls