My family and I got the chance to spend nearly 3 weeks on the South Island of New Zealand in 2019. Overwhelmingly one of the very best experiences in my lifetime. We camped, hiked, saw the sights, went to museums and cultural centers, explored some urban destinations, and ate like kings and queens.
Like you, we didn't do much planning...but had a couple of "must see" destinations in mind. The rest was based on studying maps when we got there and recommendations by RV park managers and strangers we met along the way.
Some of my pictures of our NZ trip are uploaded into the user gallery here.