Although not advertised as a "tour," this ride offered some of the most spectacular scenery we saw in New Zealand. The ferry is a 2 1/2-hour trip (in the Lynx's case, a catamaran as opposed to a normal ferry), and it takes you from Wellington, across the Cook Strait, and through the Marlborough Sounds, at the north of the South Island.
Not only were there great views, but it was much cheaper than flying to the South Island--we got our tickets at a discount, so I am not sure how much they are normally.
The ferry offered anything we wanted: if we just wanted to sit, there was food, a TV, room to walk around, and even a land-line telephone, but we spent most of our time in the back of the ferry jockeying for position to see who could get the best photo. And I, being notoriously seasick, didn't feel sick at all until I tried to write in my journal, because it was a very smooth ride.
When you get off the ferry, you are in Picton, which has a few backpackers and a few things to do, but it also has all the major bus lines that meet the ferry and take you on to wherever you want to go, whether it is down the east coast to Kaikoura like us or across the top of the island to Nelson and the Abel Tasman, one place I would have loved to have seen.