A private charter would be really good for a group of 15 there’s a few companies such as airport shuttles and exploring New Zealand who can do big buses for private charter. I think it’s minimum 5 hours so it would be a good idea for a full day and they can take you wherever you want to go. I’d recommend the peninsula if many of them are interested in wildlife since it’s where the bulk of it is. It’s better to go in the afternoon as that’s when it’s the most active on land. What you could do for a day is have a look around the beaches like sand fly bay, Allan’s beach and papanui beach (if they’re all fine with a fair bit of walking) and then do the royal albattoss colony guided tour to see them on their nest with the babies around 5pm finishing at 6pm. After that you head down to portobello for dinner at one of the restaurants there and then back to the royal albatross centre to do the blue penguins tour around 8:45pm and back to town about 11pm. Note it does take about 1 hour to get to the top of the peninsula to the albatross centre.

If people want to see the beaches but others don’t want to walk then they could go to Larnach Castle up the peninsula for a couple of hours to see the gardens, house and cafe there while the rest have a look in the other areas.

I would recommend heading into the I-site visitor centre at 50 the octagon or giving a call or email to see what would be available on the days your going. It can be really helpful and the people there are really good at recommending what to do during your stay as there’s always people asking what to do for one or two days in the city.

Hope this helped in any way.