Alternatives to Milford, Kepler, etc.

I am in NZ for work but managed to get couple of weeks off. I managed to book Abel Tasman and Heaphy for next week. I really also want to experience some hiking on the mountains during summer on the South Island but of course the main Great Walks have been fully booked for months. Are there alternatives? I wouldn't mind going south than Nelson area for some great views!

If that matters: I have been hiking most my life back home in Brazil on mountains/beaches/jungle areas but not really used to multi day hiking. Although I did my research and I am feeling confident for Abel Tasman and Heaphy but I know they are on the "easy side".