I was there in '07, driving around and living out of this for a month:
We did Tongario - including the climb to the top of Mt Ngauruhoe - one day, and then the following day climbed to the top of Mt Ruapeho on our own, which was pretty challenging. Also, sand tobogganing, dolphin watching, black and white water rafting, mountain biking, sea kayaking, river canoeing, heli hiking to the top of Franz Josef glacier (not Fox ) and walked the Kepler track in two days.
Truly spectacular country and nature. Being able to sleep in your van by the sea, take a morning swim, drive for a bit, take a hike up through a tropical forest and ending up on a ridge with a view of snow capped peaks - pure bliss :-)