The Tour of Cadbury Chocolate factory is very over rated and very over priced. You get to see only a little of the packing areas and what they called a 20' "chocolate" waterfall. It really is chocolate, but not very impressive.
Unique Suggestions: Don't expect much, be prepared to wear full face coverings, hair nets and booties.
Fun Alternatives: Tour the railroad station, Ovelston, or Laronoch Castle.
According to the Guiness Book of Records, Baldwin Street, in north Dunedin is the world's steepest street. I've classed this as a tourist trap, just to have something in this category. It is not really very insidious and the street has some nice verge gardening that neighbouring streets don't. It's worth going to have a look at if you're in the area, just to say "been there, seen that."
Unique Suggestions: Walk up the street - I didn't because I had to race off to the peninsula... and it was such a nice day!