Botantical Hill is a good walk to do to get a view of Nelson City. It's a pleasant amble up a wide track and doesn't take too long (maybe 15-20 minutes). The great thing about it is that you can access it from the city centre which is perfect if you don't have a car and are staying near the city.
I'd definitely recommend taking a brisk walk up Botanical Hill! Good for blowing out the cobwebs if you are in town on business or holiday.