Mountain Biking
The Whakarewarewa forest is reputed to have some of the best mountain biking trails in the country. 
I guess since they are hosting this: (click to open in new window)
it was worth a try. I was very impressed.  The trails are dedicated to mountain bikes, so there are no pedestrians to interfere with the fun.  Also, they are one-way, so there's never a concern about oncoming traffic.   Some groups whinge about the impact of mountain biking on the forests, but if the traffic is managed, and confined to specific trails, the two can coexist.    Confining the traffic to specific trails is the key point;  if you let everyone go every which-way, then yes there would be damage to the forest floor.  Of course Kiwis are more respectful of their environment than others, so I expect this helps.

 
 
.

 

 
.

 

The trails were very well marked.
.

 

Some parts were more fun than others.

 
Wonderfully dense forest.
.

 

 
I rented a bike from Ed, who was really cool & friendly, & gave my bike a thorough going-over before renting it to me.  Here's one of his sites: