It's not until we've completed our first rapid that our guide, Tim, explains why there's a lettuce on board.
I'd presumed it was someone's idea of a joke, an allusion to Tom Hanks' inanimate mate in Castaway perhaps, or something to throw overboard before a tough rapid to see what we were in for.
It turns out Tongariro River Rafting boss Garth Oakden is fly-fishing down the river and has phoned in for sandwich supplies.
By the time we reach him, my raftmates and I - perfect strangers an hour ago - are chummy cheeseballs high-fiving our paddles together.
White-water rafting has to be one of the most fun things you can do on water, and you don't get much more consistent fun than the Tongariro. Clear water flows through a scenic volcanic gorge, taking in 60 rapids, each one just a minute or two apart.
Most of these Grade 3 rapids are perfect for beginners like me on a guided trip, but part of the thrill is not knowing what's coming.
"That one's called 'The Bitch'," says Tim after a particularly bouncy ride that leaves us soaked and laughing.
It's to his testament we don't slam into the cliff face afterwards - the Tongariro is one of New Zealand's most technical rivers.
As we approach another rocky obstacle course, Tim calls out instructions for us to move left or right, leap to one side of the boat, paddle forwards or backwards.
Sometimes it looks as tricky as a lorry doing an eight-point turn, but all we have to do is listen and obey his command.
Part of the fun is getting into minor scrapes. No one falls in until the end, but along the way we spin on rocks, graze walls and, after a seemingly pointless exercise where we paddle furiously against a rapid, hover in an eddy on the lip of the gushing water.
Later, Tim invites a raft-mate to sit at the front with her feet over the side - by the time we've rafted the rapid she's flat on her back with her legs in the air.
It's brilliant fun, but when the water stops fizzing, the laughter dies down and our raft plateaus out, we're gliding into stunning scenery: wide, glassy pools flanked by white cliffs and ancient beech forest.
If you're not into rafting you'll still find plenty of heart-pumping activities in the Taupo region, whether it's taking advantage of the world-class trout fishing on the Tongariro, skiing on Ruapehu or taking a ride on the Hukafalls Jet.
And while it's Taupo that gets most of the tourism glory, in recent years the southern lakes areas of Turangi and the tiny thermal town of Tokaanu have emerged as destinations on the up.
Turangi, where I'm staying, was once dogged by petty crime and flat house prices, but now has a big supermarket, busy information centre and nice cafes.
On the drive from Taupo airport to Turangi, I stop at the Licorice Cafe in nearby Motuoapa for bacon and eggs; that night I overindulge at the Bridge Fishing Lodge on delicious chowder, venison and creme brulee.
The next night it's scallops and fine wine at Kaimanawa Lodge, overlooking the lake.
Turangi still has a leisurely, historic charm but is getting more competitive with its lakeside cousin. The locals reckon Turangi is the Wanaka to Taupo's Queenstown. My fishing guide says people talk about doing things in Taupo; in Turangi they do them.
With that in mind, I book in for the Tongariro Alpine Crossing, a full-day's trek up the mountain promising spectacular views, otherworldly terrain and a sore butt.
It's an 18.5km walk, and although you can choose whether or not to deviate from the main track to the summit's 1978m, there are plenty of other thigh-burning climbs for the seriously fit - including a hike up the slippery scree of Mt Ngaruhoe.
As a lone walker I decide to stick to the main trail.
Tongariro Expeditions takes me to the base of the mountain, where hiking gear is provided, but after that I'm on my own.
Official guides are not allowed on the crossing because of land ownership, but there are negotiations under way to make the walk safer. (An American tramper died as recently as 2006, the second in a year. After that the word "alpine" was added to the crossing's name to spell out what people should expect.)
It's now 7.45am and I'm glad I hauled myself out of bed for the bus an hour and a half earlier - the light is spectacular. Shards of sunlight peak through gaps in the mountain, which looms dark and imposing ahead. Walking alone isn't as terrible as it sounds. Even now there are hundreds of eager trampers setting off in small groups, like ants snaking up the mountain. It's enough to put my mind at ease, alongside the knowledge that my cellphone has coverage most of the way across.
The first hour is deceptively serene; a gently climbing path that cuts its way through twisted black lava as a waterfall bubbles away alongside it. It's no preparation for the "devil's staircase", a 45-minute, well-named climb that opens out to spectacular views on one side and wide, dusty craters on the other.
The air is incredibly dry, and by the time I ascend to the base of the next climb, absolutely freezing. Despite the cloudless blue sky and omnipresent glare of the sun, the wind whipping across the ledge is so cold I'm forced to stop and pull on all my thermals.
As I climb the narrow ledge to the top of the Red Crater, I'm relieved it's not winter, when trampers are often forced on to their hands and knees to get to the top. Even today, there's the sense that this dormant beast could turn into the White Witch of Narnia at any moment.
But Tongariro's austere beauty makes this all worthwhile. The climb opens up to magnificent views of the valley and the surreal Emerald Lakes, large sulphuric pools that look like splashes of bright blue paint have dropped out of the sky. A walk across the moon and I settle in at the eerie Blue Lake for lunch, amazed at the lack of life here. Shouldn't there be windsurfers? Boaties? Birds?
This alpine landscape is so barren that even this tempting, shimmering lake looks alien.
Life reappears on the way down and it's a relief to feel the sun on my face.
The colours warm up too - an assortment of muted pinks, dusky greens and rocky purples.
After another food break at the Ketetahi Hut, the scenery changes drastically again. Violent rock formations make way for fields of silky tussock grass, mauve creeks and, finally, gorgeous bush that never seems to end.
Even after the strenuous climbs of the morning, this is the hard part. My knees ache. My spirit is spent. I chat idly with a fellow walker and 30 seconds later, I've forgotten what was said.
It will be another few days before my body unstiffens from the 6 1/2 hours of walking, climbing and sliding. But the memories of the trip will stay with me forever.
Rebecca Barry flew to Taupo on Air New Zealand and stayed at the Bridge Fishing Lodge, Turangi, care of Destination Lake Taupo.
GETTING THERE: Air NZ flies return to Taupo three times a day, Monday to Friday and twice a day on Saturday and Sunday. See www.airnz.co.nz.
WHERE TO STAY: Bridge Fishing Lodge can be contacted at (07) 3868804 or www.bridgefishinglodge.co.nz.
WHERE TO EAT: Oreti Village Restaurant is at (07) 3867070 or www.oretivillage.com.
WHAT TO DO: Tongariro River Rafting can be contacted at (07) 3866409 or www.trr.co.nz.
Tongariro Expeditions is on the web at www.tongariroexpeditions.com or ring (07) 3770435. To get in trim for the Tongariro cross, or just have fun, try wall climbing with Extreme Backpackers. See www.extremebackpackers.co.nz.
For information about Hukafalls Jet see www.hukafallsjet.com or ring (09) 374 8572.
FURTHER INFORMATION: For information on Taupo see LakeTaupoNZ.com.