Can't help on the excursion but we stopped in Picton a couple of years ago while on the Radiance of the Seas and I wasn't impressed with the small port town so I'd suggest booking some kind of excursion while there. We got off the ship and walked around the town for a bit, shopped, had lunch and then headed back to the ship.
Good to know. My cousin still wanted to go on the Whaling Station Excursion. I booked myself on the Taste of Marborough, wines, cooking oils and chocolate.