Hands-down the best way to see the country is to rent a sweet RV, especially for longer trips. Unlike in the U.S., where RVs are often corralled into concrete "parks," New Zealand has a "freedom camping" ethos. That means you can pull off the road almost anywhere, as long as your rig has a toilet on board and you're not in a city or the restricted zone of a national park. RVs also make great rolling gear garages (kayaks, bikes, packs, raingear ...), and having a refrigerator means you can cook green-lipped mussels at sunset. Two planning suggestions: prepare mentally to drive a large vehicle on the left side of the road, and budget for a pit stop or two at one of New Zealand's legendary lodges.
If you have three or more weeks, it's worth touring both islands. Most international flights land in Auckland, and if you head (mostly) south from there, it's roughly 1,200 miles and a half-dozen climates—from subtropical coast to high-alpine glaciers—to Fiordland National Park. But a truly epic adventure might have you logging 2,500 miles or more. On the North Island, spend some days on the Pacific Coast Highway, lazing on the beaches of the Coromandel Peninsula or surfing the beach breaks in Tauranga. Turn inland on Highway 33 and hit Rotorua's famous hot springs and mountain-bike trails. Eventually you'll want to roll through the Hawke's Bay wine country to Wellington, the antipodean San Francisco and port for the Interislander Ferry, which crosses Cook Strait in three hours (prices vary by vehicle; interislander.co.nz).
The road tripper's route on the South Island is Highway 6 along the wild west coast and the Tasman Sea. It's oceanside car camping out there and an easy three days to the Franz Josef and Fox glaciers (guided glacier walks at both spots; franzjosefglacier.com and foxguides.co.nz). Inland over the Haast Pass is the adrenaline-sports capital of Queenstown, the birthplace of commercial bungee jumping and guided canyoneering adventures (see "Get Amped"). You could play here for a week, but it's worth cruising farther down the island to Fiordland National Park, where the road dead-ends at Milford Sound and sheer rock faces plunge into an inlet full of whales, seals, penguins, and dolphins.
Your RV provider: United Campervan (campervan.co.nz), which has a wide selection of models (daily prices vary depending on the season and vehicle), or Christchurch-based Natural High, which offers RVs and any combination of kayak, bike, and camping-gear rental (naturalhigh.co.nz).