The bicyclists skidded to a stop along the Kauai path and stared out to sea, transfixed by a magical marine dance of humpback whales.

Several massive whales — humpbacks can be as long as a bus and weigh 80,000 pounds — were breaching off the Hawaiian island’s shore, leaping out of the water and crashing back down with mighty splashes. Other humpbacks undulated through the waves, glistening and spouting jets of spray, some slapping their massive tails against the water.

A dozen of us — bikers, strollers and joggers — stopped and stared in awe. “Just another day in paradise,” murmured one tourist, leaning on his rented bike.

The humpbacks quietened down after a stunning 10-minute natural show. And we all resumed our outings along the paved oceanfront path, one of Kauai’s man-made treasures.

If you want a little exercise after lolling on Kauai’s lovely beaches, head for this multiuse path, called Ke Ala Hele Makalae, on the east coast of the island. It’s an easygoing, mostly flat route that partly follows what was once a sugar-cane railroad line.

In the Hawaiian-language, its name means “the path that goes by the coast,” said Randy Black, a semiretired physician and avid bicyclist who lives on Kauai and is one of the movers and shakers behind the path. He’s the executive director of Kauai Path, a nonprofit group that advocates for pedestrian/bike trails all over the island.

“You can walk or bike and see the spouting whales — and it’s free,” said Black.

Eventually it’s planned that Ke Ala Hele Makalae will stretch about 18 miles along the east coast of Kauai, from Lihue, the island’s main town, north to Anahola Beach Park.

So far, about 6½ miles of the path has been completed in two separate sections, 4.2 miles around the town of Kapa’a and 2½ miles at Lydgate Beach Park, south of Kapa’a. Paving began earlier this year on another 2.1 miles of the path that will connect those two sections. Managed by the County of Kauai, the linear park is being built mostly with Federal Highway Administration funds and the sweat equity of community volunteers who have been working to create Kauai bike/walking paths for about 15 years.

On a midwinter visit, I rented a bright-red bike at Coconut Coasters, a small, friendly bike shop just off the trail. The bike, with three speeds and a basket, was like the old-fashioned ones I used to ride as a kid, perfect for cruising along the easygoing trail.

I could have biked the whole trail round trip, at a leisurely pace, in less than an hour. But why go even that fast? Take a picnic, take a beach blanket, take a book, and stop along the way at white-sand beaches or on bluffs with endless views of the Pacific — and, if you’re lucky, humpback whales when they winter in Hawaiian waters.

For walkers, the most scenic part of the trail is north from Kapa’a for several miles (I used the handy trail access point right by Coconut Coasters). The northward trail soon leaves buildings behind, hugging the coast for several miles and skirting beach parks. Bikers could easily do the whole trail, both south and north from Kapa’a for an easygoing, and blissfully scenic, round trip of just over 8 miles.

I parked my bike and lingered with the locals at Kealia Beach Park, a half-mile stretch of white sand that’s one of the busiest beaches in Kauai; lots of locals live nearby and Kuhio Highway, the main road on the island’s east side, passes by the beach. Guys pulled their shiny pickup trucks up to the sand, reggae music pumping out of the cabs as they unloaded their surfboards. Teenagers bodysurfed in the tumbling waves, looking as sleek as Hawaii’s monk seals. Along quieter stretches of the beach, families settled down on the sand, basking in the sun and tropical breeze.

Lifeguards are stationed at Kealia, making it safer to swim at than Kauai’s unguarded beaches. (There were eight drowning deaths in January and February in Kauai, both tourists and locals, so respect the ocean’s waves and currents.)

Pedaling happily north with a sprinkling of other bicyclists and walkers, I reached the secluded Donkey Beach where the paved trail dead-ends on a bluff above it. Once this was a popular nude beach, about a 15-minute walk from the nearest road. But the trail reached here in 2010, making Donkey Beach just a five-minute walk. That’s brought more people — and bathing suits — to the beach.

Donkey Beach was nicknamed after a sugar plantation company’s mules that once grazed above the beach (its Hawaiian name of Paliku is rarely used). Now some luxury houses have sprouted high up in what were once fields.

Down on Donkey Beach it feels remote and wild, a windswept quarter-mile of sand battered by waves and currents that usually make it dangerous for swimming.

Heading back to Kapa’a, I cruised along the trail in a balmy wind and occasional raindrops, watching dark rain squalls sweep offshore. No more whales, unfortunately.

Yet Kauai wasn’t done with showstopping scenery. A rainbow started shimmering above the ocean, its colors growing stronger and stronger as I watched. It arced gracefully across a bay, with both ends visible. I didn’t need to make a wish upon a rainbow; it couldn’t get better than my afternoon on Ke Ala Hele Makalae.