Lived there for a year and loved it…
just my 5p worth..
Tongariro Crossing & Kayaking (hundreds of places to do this) are well worth it
Milford sound and franz joseph IMO were over hyped tourist traps.
Milford sound is a LONG WAY from the nearest MAIN town (if you take the dead te anau out of the equation) IF you do go to milford do not do the one day trip from Queenstown, its a ball breaker, I would advise to either A- stay in te anau OR stay at the hotel down in milford sound.
Franz Joseph – i did the half day walk and it was ok but was not "OH MY GOD THATS AMAZING" if you have the cash take the helicopter.
i thought Wanaka was too dead, queenstown is full of 'dudes' but still worth it, get the cable car tot he top of the mountain and the view down is amazing.
Lake Tekapo is veyr nice and chilled, there are some tiny lodges you can/could hire at the time a stones throw form the lake.
Taupo is pretty cool aswell
its just amazing driving round the country.
enjoy!
ps- i'd hire a car then get the AA guide book and book b&b's etc ahead of your route,