So the itinerary would look like this :
1) Landing 2 pm at CC, then road towards Arthur's pass, night at Arthur's Pass
2) Visiting Arthur's Pass, if time have a look at Paparoa, night at Greymouth
3) Visit Paparoa and/or the first glacier, night at Franz Joseph. For Paparoa we're gonna see Punakaiki Rocks (at high tide hopefully) and try to squeeze in Motukieke Beach Walk at low tide. Anybody done this walk ? It's Scott Cook's favorite beachwalk, but it would be nice to have some feedback from New-Zealanders.
4) Do Rob Roy Glacier if there's time (other option : second glacier), night at Wanaka. Apparently Rob Roy is a must-see, but the road leading to it is pretty rough, is it corrugated and how fast is it safe to drive on ?
5) Road towards Mount Cook, day walks around lake Pukaki, night Mount Cook. In the area I've seen pictures of Clay Cliffs of Omarama it looks really amazing.
6) Day walks at Mount Cook, night at Mount Cook. We're gonna do Hooker Valley Track, THE walk I want to do the most in NZ, I hope it's gonna be a clear day. On NZ Frenzy they advise you to do Ball Flats Road Route on a grey rainy day instead, you guys agree ?
7) See lake Tekapo, Taniwha paintings, night at Oamaru

9) Visit Curio Bay, night at Te Anau
10) Visit Milford Sound night Te Anau. I'm not sure if the boat-trip is really worth it, between the boat and adding walks like Humboldt Falls, Gertrude Valley or Tutoko Valley Route which is best ?
11) Road towards Queenstown, day walks in the area. Is it worth it to do Routeburn until Flat Hut of Falls Hut ? Or is it better to do Lake Alta and Shadow Bassin Lookout ?
12) Taking off from Queenstown in the afternoon.
I realize that everything's a little rushed, but it's not entirely my choice, I have to compromise with my travel mates. Some days have 3 hours of driving at a time, some have 4 hours in total, but I'm sure it's gonna be worth it, and the roads are quite scenic apparently so it's not just boring straight lined roads =) If you've got any tips on secluded spots and walks, that would be great, I found Tutoko Valley thanks to Wayno on an old topic for example. Thanks in advance for your help !