If you want to explore all that New Zealand has to offer you will need to cross water at some point to visit both islands so will either need to use a boat or air transportation.
Other than that, you can hire a motorhome (RV) vehicle and cruise around at your own pace, visiting the parts you want to.
New Zealand also has a train system you can use if you don’t want to hire a car or other transport.
Alternatively, you could hire a bike/hike around the countries.
There are many ways to explore New Zealand.