2nd tramping trip to NZ - any recommendations?

Hey guys - was hoping you might be able to give me some advice. i'm coming fromt he UK to do my second trip to NZ in Jan/Feb. Last time I did the Kepler, Whanganui River and Tongariro Northern Circuit. It was an amazing trip and I saw so many stunning and varied types of landscape. I'm coming back and want to do again maybe three decent tramps, and some smaller ones. Again - I'm really hoping to get some real variety in my scenery. I have a month, but also want to spend about a week and a bit with family and friends. I was thinking the Cape Reinga Walkway (I have family in the Bay of Islands), the Lake Waikaremoana Great Walk and some time in the Abel Tasman National Park. I'll probably have time for maybe one more big tramp (hopefully two!) before I finish my trip in Christchurch. I'd love to get something completely different to what I'll have done so far - maybe a bit more off the beaten track... and hopefully reasonably challenging - any recommendations? I was thinking The Heaphy Track, or tramps in Arthur's Pass National Park or Nelson Lake National Park. But maybe people have better ideas? Also, given that I'm definitely going to be doing the Abel Tasman and finishing in Christchurch, would I be better off maybe skipping Lake Waikaremoana and doing Nelson Lake National Park instead? I want to come back for a third trip sometime and do the South of the South Island more extensively, so I think on this trip I probably don't want to go too far south of Christchurch... I'd be really grateful for any ideas you might have!