The Coromandel offers many spectacular places to go walking or hiking.  Some of the highlights are:

Kauaeranga Valley :	Located 14k inland from Thames.  This area offers numerous trails, complete with camping facilities.  Bunk house accommodation is available at the Pinnacles Hut.
Castle Rock:	This strenous hike from the 309 road to the top of Castle Rock can take up to 2 hours to complete, but affords great views of the Coromandel and the Hauraki Gulf.
Kauri Grove:	The 309 Kauri grove is a short walk (15 minutes) but allows you to discover Kauri trees - one of the main reasons that this area became developed.
Otama Forest:	The Otama Forest walkway is just outside Kuaotunu and crosses farmland.  There is also an abandoned goldmine that has glow-worms and old mining tracks to follow.

For a fairly comprehensive list of walks in the region, take a look at the Waikato area on the Department of Conservation Website.