Abel. Tasman. (f you like hiking or kayaking that is) I would at least rent kayaks for the day at motueka. I had never gone kayaking in my life, and now I'm hooked from that one experience.
The drive to takaka is amazing. If you take the camper across to the north by ship (will run you $200 at least), I recommend the less-used Queen Eliz. II highway to picton from nelson. It is full of hairpin turns that will have you gripping the wheel with some fervour, but the exended time and scenery gained is really worth it. (I guess it depends on the size of your camper!)
Even if you can't get onto the Milford Sound trek, you should still drive to the town! It's pretty stunning as well, and you could have a day picnic on the way there (stop at the hollyford area or such for a nice spot) Steep driving though, but this is south NZ, get used to it!

The waterfalls on the way alone are worth it.
I think alot of people used the new 'spaceships' (orange nissan van things that have lots of cool gagedtry), and I would give that a look for sure. Otherwise Keas and other generic campers are going to be pretty straightforward, so find the best price with those 'uns. I don't have much experience with campers myself, so maybe others would be more helpful here.
"don't miss" HELL PIZZA! Oh my word, what a great pizza they make. Oh yeah, mac's beer - especially the cloudy weissbeer.
Tongariro park will blow your mind - If you do just the one day in-out trek, plan to spend some time wandering around the 'mount doom' area, many people just hike in, take pictures, and hike out - the area around it is what make it what it is - a buildup of many diverse landscapes that point to the culmination of evil that is the sulfurous mountain. (I must say that the three day trek is really worth it, go counterclockwise for the best kind of "Lord of the Rings" dramatic suspenseful buildup)
Wellington is better than Auckland. Ha. So there, I've started a flame post.

Have a great time, it's SUCH A BEAUTIFUL COUNTRY! I know I sound like a teenage girl on MSN here, but holy crap I get excited when people talk about New Zealand, what a great country. Don't forget to post and let us know how you enjoyed it!
Cheers, MacK