I did both islands in about three weeks. Rented a van from Kiwi Kombis,
https://m.facebook.com/100063606802623/ which was pretty fun. Not sure how it would be for a couple, it was snug for sure. But maximum style points.
I'll second Abel Tasman, I did a trip where they ferried me up the coast and I sea kayaked back down over three days stopping at huts along the way. They moved my pack each day so it was pretty easy.
Milford Sound - I did the charter flight out and back as a splurge. Amazing scenery.
Nelson was cool. I spent a few days in Auckland at the beginning to get over the jet lag and time change. I regret not making it to Christchurch and the top of the north island. Wanaka was great. All in all, having a camper van meant I could have a pretty loose itinerary. If I heard about an interesting hike or something fun I could just call an audible. Jet boating was fun, maybe not the best value for money.
The ferry was easy the days I did it.
In general, I did a bunch of hiking and sightseeing and just general relaxing.
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