Did you see the movies? If so then this is a fun day trip, 2 hours from Auckland. Highly recommended! You should book in advance!
Hobbiton
Near Hobbiton, this is an afternoon walk and boat ride through spectacular caves. You can do this and Hobbiton in one day. Again booking is advised.
Waitomo Caves
Sounds silly but I recommend this, they actually put on a great show. About 1 hour north of Auckland.
Sheep World