It's hard to put Canterbury & West Coast into one category since it's such a huge area but being somewhat geographically close it does make sense. The variety of scenery in these areas is just phenomenal - starting in the far east with the seaside settlement of Kaikoura where snow capped mountains tower directly above the sea and it's host to an amazing diversity of sea life including whale watching.
Travelling inland to cross the Divide, there's the unusual Castle Hill, a Middle-earth location if ever I've seen one and a spot I wish I had spent more time and will definitely visit again. From here it's on to Arthurs Pass - one of the great NZ roads to drive and if you have time to stop the walk to Devils Punchbowl Falls is worth the hour detour. If you're lucky you may even spot ke at the Otira viaduct, an amazing feat of engineering that has made the journey from east to west much safer fro all travellers.
One of my unexpected favourite stops on the West Coast is Hokitika Gorge - the colour of the water here has to be seen to be believed and the famous West Coast sandflies too! Further down the coast there's my personal favourite spot in New Zealand Lake Matheson - where the reflections cannot be beaten. I've visited here twice and I know I could go again and again.
Heading back through Haast Pass towards Wanaka there are an abundance of beautiful waterfalls like Thunder Creek Falls to pause at for a few minutes and appreciate once more the amazing clarity of the water in this part of NZ.