Nelson is a great starting point for a South Island adventure.
Locally you have Picton and the Abel Tasman National Park.Not to mention the Nelson Lakes National Park,which is home to majestic peaks,beautiful secluded lakes,and tons of walking routes.
You can also head off further afield and explore other South Island attractions,including:
Milford Sound
Mount Cook(New Zealand's highest mountain)
Wine tasting in Central Otago
Cities including Christchurch and Dunedin