Griffin, if you're going to be in Rotorua anyway, you may as well do some hiking there and head over to Tongariro and maybe Ruapehu as well. Haven't been there myself, but I know there are a lot of tracks worth walking. For the South Island, get to Wellington, take the ferry over to Picton, pick up a rental car and head for the Marlborough Sounds area and the Abel Tasman. I think you could do it all by bus, but a car would be more convenient. I think you can get a car in Picton, but I'd check in advance. I'm pretty sure there is shuttle service from one end of the track to the other. If I remember right, you can hire kayaks and cruise around the islands. I did a quick car camping trip up that way and it's great. From there, I'd head down the West Coast, stop at Fox Glacier, do a walk there (not sure what shape the glacier is in now, though); then head south towards Te Anau, Queenstown and Mt. Cook. If you have time, you could go all the way down to Fiordland, but you'll find plenty to see, even if you don't get there. One thing to do is drive out to Milford, even if you don't do the track and take a boat ride through the Sound. I did that after my trek and it was well worth it.
FYI-if you belong to the AAA, you can get maps and such from the AA of NZ; otherwise, hit the local tourist bureau office. They are pretty helpful.