I've been twice (8 weeks in total), but only in summer (their summer!)
In the south island, the climate is pretty similar to the UK. So you might get some clear dry days and you might get some (lots of) damp, cloudy, cold days. You COULD do all the kind of outdoorsy stuff you like, but it would be like doing it here.
If you like all the outdoors stuff, you'd be mad not to go to the south island - the southern alps, Lake Tekapo, Queenstown/Wanaka, the fjords, the stunning west coast (with glaciers), the Abel Tasman park... It's a truly amazing place (and I've only listed some of the main tourist stops, there's a lot more to it than that). But you wouldn't necessarily see all the amazing views etc, because the weather just might not play ball.
In the north island, particularly the top of the island, it's likely to be milder (high temp in July is 14c) but still a high chance of it being wet (average of 15 days' rain). You might get nicer weather north of there (Bay of Islands etc), but you wouldn't go all the way to NZ just for that.
You say you don't really want to travel around... Not much point going all the way to NZ and not travelling. It's a very diverse country and there is so much to see. Travelling is very simple there, and is actually a joy. The journey from ChCh to Queenstown, for example, takes the best part of a day, but is a stunning drive on lovely roads with next to no traffic (although maybe not as nice in winter). But honestly, I really wouldn't go that far and then not move around.
Have you considered Canada? Some truly stunning scenery and northern hemisphere, so nice weather more likely in July. You could fly to Calgary, spend some time in the Rockies then get the train (I forget the name of it - Rocky Mountaineer perhaps) back over to Vancouver. Then spend some time there/Vancouver island. Or East coast area - Montreal/Quebec and surrounds.