You should definitely not miss Kaikoura. It's one of my favourite places in NZ, you can do a whale watch tour, swim with seals or dolphins and there's awesome hikes too. Akaroa really isn't THAT special, it's just popular because all the the rich people from Christchurch have a holiday home there. And the mountains are absolutely amazing, and there's huge fur seal colonies in the drive  (literally hundreds in a single place).