Of American-English origin, the colloquial phrase a walk in the park denotes (the type of) something easy, effortless or pleasant.

This phrase occurs, for example, in Inflation hits small firms: ‘Covid was a walk in the park compared to this’, by Sarah Butler, Jasper Jolly and Joanna Partridge, published in The Guardian (London and Manchester, England) of Thursday 19^th May 2022:

Notifications arrive every day at Loveone, an Ipswich gift shop, heralding price increases of 5% to 10% on products it sells, usually adding a couple of pounds to the sticker price.
“If I don’t get my orders in by a certain date then I will have to pay more,” says Cathy Frost, who has run the shop for 15 years. […]
[…]
“I’ve been here 15 years and went through Covid but that felt like a bit of a walk in the park compared to this,” Frost says. “We knew we were in lockdown and that we would come out.”

The texts containing the earliest occurrences of a walk in the park that I have found indicate that this phrase originated in golf slang—these texts are as follows, in chronological order:

1-: From A Caddy’s Compendium, by Margaret Erskine Cahill, published in American Speech (Durham, North Carolina: Duke University Press) of April 1937—here, there is some reference to the literal sense of the phrase a walk in the park:

A walk in the park is their facetious way of referring to a nine-hole round.

2-: From The Kansas City Star (Kansas City, Missouri) of Monday 28^th June 1937—I have not been able to identify ‘the Commentator’:

CADDY LANGUAGE TRANSLATED.
“Bag Rats” Call 18-Hole Round a “Loop;” Cheap-Skate Player a “Chirp.”

The golf caddy has a picturesque language all his own, according to the Commentator. He calls his fellow caddies “bag rats” and an 18-hole round is a “loop,” while a 9-hole round is a “walk in the park.” A “chirp” is a cheap skate player who doesn’t tip, while a poor player is a “wood butcher.”
A “shop rat” is a caddy who hangs around the pro’s shop trying to get more work by running errands and ingratiating himself. A badly kept course is a “pasture”; a slow round is a “creep” or a “crawl.” A typical caddy fee amounts to a “fish and a Q,” which in money means $1.25. A “shovel” is a niblick or any club used in getting out of a sand trap. When you get off the course you get into the “jungle.” Every golf pro is “the Scotchman” regardless of whether he is an Italian, a German or an Englishman.
An “ice cream caddy” earns money by caddying, but isn’t dependent upon it for a living. A “double slinger” is one who carries two bags more often than average. When business is poor caddies tell each other, “You’ll eat grass tonight,” and that, says the Commentator, means they’ll have to go without their favorite “tiger steak” or hamburgers.

3-: From Lizard or Bogeyman, by the U.S. sportswriter Jim Murray (1919-1998), published in the Los Angeles Times (Los Angeles, California) of Tuesday 22^nd January 1963—Jim Murray described how he felt when watching the final round of the Crosby golf open on TV:

I turned it on: Know what I saw? Warm sun. Greasy fairways. Guys in shirt sleeves knocking in birdies. Amateurs putting for net eagles on holes that normally make strong men consider tossing their clubs in the fireplace.
[…]
They even had a guy with a putter that looked as if it had been caught in a pipe or found wrapped around a tree limb. He looked like he came to fix the plumbing with it. He putts with it like a croquet mallet, and if he had won the tournament, I’ll guarantee you the manufacturer wouldn’t be able to keep up with orders for the rest of the year. I was just about to snap the show off. “It’s just a walk in the park this year,” I said disgustedly to my wife. “Crosby doesn’t even have any hot water bottles in his overcoat pockets.” The screen even showed a kid in surf-boarding.

4-: From the column Watts Cookin’, by Joe Watts, published in The Daily Herald (Provo, Utah) of Tuesday 6^th March 1973—the following is about the final round of the Citrus Open at Orlando, Florida, between Gibby Gilbert and Buddy Allin:

After winning the tourney Buddy called Tucker and said, “Coach, I’ve had a great feeling all week. I’m really playing good golf. Every time I hit it I knew it was going to head right for the flag.”
“When I got ahead by six strokes it was just like a walk in the park. I’ve never enjoyed a round of golf so much in my life,” beamed Allin over the phone.

5-: From a story by Bob Green, Associated-Press golf writer, about Jerry Heard, who had just won the Citrus Open at Orlando, Florida, published in several U.S. newspapers on Friday 8^th March 1974—for example in The Fresno Bee (Fresno, California):

MIAMI (AP) “Just like a walk in the park,” Jerry Heard said.
“The more you win, the easier it seems.
“Coming off a win, it’s easy to play good. The pressure is off.”