Side trips as day walks can also be difided up into 2 categories, side trip to do in between destinations, eg climbing Ossa on the day you walk between Pelion Hut and Kia Ora. And Side trip as a day walk, a whole day climbing for example Mt Oakleigh from Pelion Hut as an extra day to add to the trip.
I still don't get why people climb Ossa without a pack at all, and just carrying a water bottle and maybe a nut bar. Not for me... You go up Ossa and your pack is at Pelion Pass, you are pretty vulnerable, too much so for my liking without any gear. I understand most would do Ossa as a side trip en route therefore don't want to carry all their gear up Ossa, but I have seen many people take next to nothing with them. We did it as an extra day, starting from and returning to Pelion Hut. What a day it was too... Image
here and
here.
That issue aside, walks that may be considered - Cradle Mtn, Ossa (long way up), Oakleigh, Pelion East, Pelion West (harder), Thetis (harder), Pine Valley, some waterfalls like Hartnett...
If you wanted to visit Acropolis or Geryon or Walled Mtn or the Labyrinth, staying at Pine Valley would be required.
That's a rough start, out of the head anyway.
They're all covered in detail in a very worthwhile book by John Chapman,
here.
Another book, cheaper and only has the areas that interest you in this topic,
here.
Hope that helps.
Adam.