They're not easy to get to, but if you have an adventurous spirit and don't mind a rough road, French Pass and D'Urville Island are two of the best-kept secrets in the whole top of the South Island.
The road to French Pass splits off State Highway 6 at Rai Valley, halfway between Havelock and Nelson. It's winding, rough, and steep in places, but quite passable in a regular vehicle if you're a confident driver (check that your rental car is allowed off the sealed road). The sign at the start says "French Pass 2 hrs," and although it's only 64 km (40 miles) to the pass, this estimate is basically true. The road first climbs over the Rongo Saddle and down to Okiwi Bay through native bush; not far from here, you'll have spectacular views of D'Urville Island in the distance. Then the road crosses to the Pelorus Sound catchment and climbs along the ridge separating the waters of that sound from Tasman Bay to the west. Small side roads drop precariously to hidden bays such as Te Towaka, Elaine Bay, and Deep Bay.
The last 12 km (7 miles) is a dramatic drop down to sea level, skirting Current Basin before arriving at French Pass, the narrow stretch of water separating Tasman Bay from Cook Strait, which moves at up to 9 knots during the tidal run. Both the waterway and the island were named for French explorer Dumont D'Urville, who navigated through the pass in the 1820s when it was uncharted by European cartographers. D'Urville Island is on the far side of this stretch of water, and it's a fabulous spot where you can get a sense of what isolated coastal New Zealand is all about. Plan to stay two nights as it's a long drive either way.
Between Okiwi Bay and French Pass there are no facilities—no gas stations, bathrooms, or cafés—so come prepared. Public facilities at French Pass are limited: a basic toilet and a small service station with petrol, diesel, and essential supplies, open only during limited hours.