If you decide the inland route, Murchison is a worth while stop for white water river rafting.
The other interesting stop is Maruia Springs, a japanese-style hot springs in the Lewis Pass. Great scenery and very natural place. We skipped the big, swimming pool -like hot springs in Hanmer Springs and stopped at Maruia. The owners from Amuri Estate (
www.amuri.co.nz) recommended it, also worth a stop for their excellent olive oil.