You are right, it can be difficult deciding what to prioritise when visiting New Zealand, as there is just so much to see and do!
You are also right that if you book on a guided tour, you will stick to a scheduled itinerary however this can be very beneficial, as you don't have to worry about directions, you can meet other like-minded travellers and you usually have the driver giving excellent commentary throughout the journey. There are so many different tour operators in New Zealand that you simply have to search for one that caters for your needs. You decide on the places you'd like to visit and the chances are, you will be able to find a tour company that visits those places!
However, if you would prefer to hire your own car or campervan, there are also plenty of hire companies that will provide you with a vehicle and a map, so you can go where you want and when you want. There are usually plenty of car rental companies at the airports so whether you are flying in to Auckland or Christchurch, you can always organise your car hire when you get here.
It's difficult to recommend how much time to visit for, as that will depend on your budget and how much time you can spare. My advice would be, if you can, to make your visit no less than 4 weeks. There is so much to see and do and I think to get the most out of your visit, you should allow at least 4 weeks (however if you can only spare a couple of weeks, there is still plenty you can squeeze in!). If however, you decide to come between June and October you will be visiting during the ski season period and so may wish to spend longer somewhere like Queenstown in the south island and fit in a few extra days to go skiing or snowboarding.
The key places I would recommend visiting are as follows:
North Island
Bay of Islands (beaches, watersports, boat cruises. Perhaps stay a couple of nights here)
Coromandel (beautiful beaches and scenery. You could easily spend 1-2 nights here).
Auckland (New Zealand busiest city. I would recommend about 3 nights).
Rotorua (the geothermal area of New Zealand. Spend a couple of nights here and enjoy spas and mud baths as well as some Maori culture).
Taupo (stay here if you like walking and would like to try the Tongariro Crossing, New Zealand's best one day walk).
Napier (art deco town, great for wine tasting, cafe culture and great architecture. Perhaps 1 night here).
Wellington (New Zealand’s capital city, lots of arts, theatre, music, restaurants and bars. You could certainly spend a few nights here).
South Island
Nelson (visit the Abel Tasman National Park from here, it is beautiful. You could spend a few nights here).
Franz Josef or Fox Glacier (definitely worth a visit. Take a helicopter flight over the glaciers or book on a tour to walk on the glaciers themselves. It's spectacular. You may like 1-2 nights here).
Queenstown (you can do a day trip to Milford Sound from here which is stunning. Most people who visit New Zealand love this town and hate to leave. It's the adventure capital of the world, with lots of activities but the scenery is also breathtaking and you can indulge in relaxing hot pools, spas, coffee shops and walks. It's a great place to relax or be adventurous, whichever you choose! I would recommend 3-4 nights if you can).
Wanaka (Great to visit in winter time when the ski fields are open. Perhaps 1 night here).
Mount Cook Village (Mount Cook is New Zealand’s largest mountain and the village offers incredibly beautiful scenery. It is a great place to go for a walk and I would definitely advise staying here 1 night).
Lake Tekapo (enjoy hot pools, ice skating and star gazing. On a clear day you can see Mount Cook. It's a beautiful, relaxing spot. Perhaps 1 night here).
Dunedin (lots of great architecture, galleries, museums, shops and wildlife. Perhaps 1 night here).
Christchurch (Known as the 'Garden City', spend a few days here walking around the Botanic Gardens, riding the tram, visiting the galleries and museum and visiting the shops and cafes. Perhaps 2 nights here or stay an extra night and visit nearby Akaroa or Hamner Springs).
Kaikoura (the best place to taste seafood and go whale watching. You could spend 1 night here).
I have tried to highlight some of the key attractions at each place. The times I have suggested at each destination are only a guideline if you don't have a lot of time to visit. Of course, if you have extra time, you could certainly spend longer at many of these places and indeed visit many more.
There are lots of excellent accommodation options here in New Zealand, whether you are looking to hire a campervan and sleep on the road or if you would like to book in to bed and breakfast, hotels, motels, backpacker hostels or luxury loges and apartments – the choice is yours! The country is dedicated to tourism and you’ll find somewhere to eat and sleep at pretty much every place you’ll visit.
Take a look at www.destination-nz.com and read up on the different destinations in New Zealand and the activities you can enjoy. If you hire a car, it is very easy to drive around the country and very beautiful. I'm quite sure you will have an amazing time. I hope this has helped a little. Please feel free to ask if you have any more questions!