Outside the train window, there’s a flickering reel of flowering fruit trees, lambs and swans nesting on the marshy levels. Following the Exe estuary towards Dawlish, where the railway runs along the beach, flocks of waders are gathering on the sandbanks, backed by boats and glinting water.

I’m heading for the Dart valley and the English Riviera, AKA Torbay, to explore by foot and ferry, river boat, bus and steam railway. The area promises wine, walking, seafood and an eclectic history from prehistoric cave-dwellers to Agatha Christie. It’s easy to assume a Devon holiday must involve driving, but it can be even better without. On previous trips, I’ve stayed in Exeter and toured by train, or based myself in Torquay to walk the coast path and take the boat to Brixham. This time, I’m testing the limits of what can comfortably be done without a car by staying in an old farmhouse in the countryside, half a mile from the nearest bus stop.

Bus 125 runs every couple of hours from Paignton, and the road from Four Cross Lanes bus stop to Sandridge Barton winery is laced with stitchwort and pink campion, growing under hedges wreathed in honeysuckle. Robins are singing in the apple trees and skylarks over the fields. Hard to believe I was in rainy Essex this morning and am now in a sunny Devon vineyard, ready for the midday tour. A whiff of wood smoke from the fire in the tasting cabin mixes with soft bubbles from the Sharpham sparkling wine, made on site and tasting of oak and tart apple crumble. The last taster, a young, fruity red, comes with Sharpham cheeses and local chilli jam.

Afterwards, I stroll for a mile down to Stoke Gabriel for provisions. The route is almost impossibly idyllic: a narrow track under banks of ferns and wildflowers. The only sounds are birdsong and a waterfall near the old mill. It’s a path I might never have found if I’d come by car. Along the stony beach of the big Millpool, I reach the River Shack and sit right by the water with bowls of sesame-seaweed salad and honeyed anchovies. Dozens of green and orange crabs are marching sideways over the dam nearby, caught and released by children fishing there with sacks of bacon. A heron is fishing too, far out in the pool.

Back at Sandridge Barton, the new restaurant has closed for the day, and there are only sheep in the orchard and swallows spiralling overhead. My brother joins me from Somerset on the last bus and we have a whole floor each in the pale pink, wisteria-decked Lower Well Farmhouse next to the winery. It would comfortably sleep eight in three doubles and a twin, all en suite, and downstairs there’s a slate-floored farmhouse kitchen, beamed, log-fire-warmed sitting room and a walled courtyard strung with lights (from £278 a night, three nights minimum).

Agatha Christie’s house at Greenway is roughly three miles south, beside the River Dart, so we decide to walk there the next morning. After a mile or so along a lane, paths lead down to the quay at Galmpton Creek and we pick up the well-signed Greenway trail, along a stony beach, through woods and over hills, to the gate. There are lockers to store bags and free hot drinks for visitors who reach Greenway without a car. We have simnel scones with pots of tea before wandering through the gardens. Woodpeckers yaffle above banks of primroses and strawberry flowers, lipstick-bright camellias and cascading rhododendrons. We sit in Christie’s old sitting room, where 1930s songs are playing, and stroll into woods, past mossy fountains, river views and hillsides white with wild garlic. This year, the National Trust has introduced a new half-price admission for non-members after 2.30pm (full price £17 adults, £8.50 under-18s).

The Dartmouth Steam Railway and River Boat Company runs different routes across the River Dart and sells combi and Round Robin tickets. From the little quay below Greenway, we catch the boat to Dartmouth (£12) along the wide, tree-lined river to explore palm-studded gardens and half-timbered houses. A quick ferry trip (£1) to Kingswear brings us to one end of the steam railway (£23.50) and, from the glass-walled observation carriage (£3 extra), the views are unbeatable.

The train passes wooded riverside cliffs and crosses viaducts, then climbs through oaks and neon-yellow broom flowers to reach the sandy beaches of Torbay. We end the day eating River Teign mussels on a sunny, pub-style terrace at The Boathouse in Paignton, with the foaming sea just feet away. The only problem is, once we’ve finished, the last bus has left and our options are a four-mile sunset walk or a £12 taxi. We take the taxi.

Heading to the bus stop in Stoke Gabriel the next morning, paths lead us up from the water, through a blossoming community orchard and the village churchyard, where a 1,000-year-old yew tree rests its sagging branches on wooden posts. A PlusBus ticket for the whole Torbay area costs £3.70 for a day or £13 for a week and gives you unlimited bus travel as an add-on to a train ticket.

From Paignton, we catch bus 22 to Torquay harbour. Red-legged turnstones are scampering under oyster-crusted pillars, and a new statue of Christie with her favourite dog, Peter, was unveiled in April. She was born in Torquay in 1890 and several novels feature local settings. She would have been more familiar with yesterday’s boat and steam train trips than today’s Rib ride (£37). A breezy, hour-long coastal tour takes in caves, coves and rocky arches, cormorants and harbourside seals, before flying us back across the bay with the salt spray spattering our laughing faces.

Torquay is peppered with Christie-related landmarks, including Beacon Cove, where she nearly drowned; the Grand Hotel, where she honeymooned; and Kents Cavern, which inspired her mystery novel The Man in the Brown Suit. The tour of Kents Cavern leads through a wooden door into dripping tunnels and dens of ancient cave bears, past stalactites, calcite bands and pickaxe marks. The guide lights moss-and-beeswax-filled scallop shells and holds up neolithic skulls like a cheery, enthusiastic Hamlet (£16.95 adults, £14.25 under-16s, if booked in advance).

Torquay Museum, in an elegant gothic building five minutes by bus from Kents Cavern, displays the fossilised jawbone of a 41,000-year-old human, one of Britain’s earliest Homo sapiens. You can also find stuffed birds and farmhouse kitchens, delicate Egyptian sarcophagi and pictures crafted in Torquay marble. In the UK’s only gallery dedicated to Christie, there’s also one of her fur coats and the walking stick David Suchet used to play Poirot (£10 adults, £5 under-18s for an annual pass).

There’s time before our train for a meal at No Seven, a seafood bistro with a sea-view wine bar upstairs. From the Tickled Pink Torquay gin, flavoured with raspberry and rose petals from Torre Abbey garden, through the gurnard with basil and tomato, the tempura plaice and skate wing, to the coffee with local fudge, it’s pretty much perfect.

Accommodation was provided by Sandridge Barton and travel by GWR (advance tickets from London Paddington to Torquay or Paignton from around £50 each way, or from Taunton from £6 each way), with additional help from English Riviera