When you cross from the North Island to the South Island via water way from Wellington to Picton, the ferry navigates through the beautiful Marlborough Sounds.
The city center itself is not particularly eye candy, but the surrounding sound most certainly are. Take the scenic route whenever you can, it is longer but you get a such a view! Take some time to walk the Queen Charlotte Track or part of it. If the weather plays the game, you will be able to enjoy nice views of the Marlborough Sounds. You can take a boat tour and navigate areas that you would take a lot of time to reach otherwise, and other places you would need to be allowed as they are on private properties. After a long day of walking, reward yourself with green mussels, the local delicacy.
There are plenty of places to stop in and around Picton on your arrival or before taking the ferry.
Once you arrive in Nelson, don't miss the city center of this small liveable city. It is full of warmth. There are plenty of nice shops and cafés.