Looks like another destination quandary has been solved - I agree with all the comments above, but can still recommend Rotorua, Lake Taupo and the Bay of Islands (the latter if you have time for a long drive and want to (try to) see some Kiwis of the avian kind ).

On the drive to Rotorua, be sure to stop off at the occasional 'Dairy' for delicious (IMO) icecream.

Make sure you have a map or know your route or have a GPS - there is only one freeway (aka Motorway) and then you're on winding country roads with very few signs for distances to the next place!