South Island NZ

Well my wife and I are finally going to NZ in September, 21 days in the South. We are hiring a campervan and driving around the place but I would love to get some local knowledge on good day walks in any areas of the South Island, I'm a mountain person so any that will get my into more wilderness areas, don't mind if it's a full 6-8hr walks as long as it's remote enough to experience a bit of NZ's wilderness. I'm hoping to be back to do two hikes early next year but this one is more a holiday. Any help appreciated