walk in the park

a walk in the park
(something that is very easy to do, and usually pleasant) — легче легкого; раз плюнуть; делать нечего (= очень простая работа); проще пареной репы

.....He's used to hard physical work - this is a walk in the park to him. (CALD)
.....The test was a walk in the park because I studied really hard for it.
.....Talking about his character in the movie Captain America, actor Sebastian Stan said that all of his actions are like a walk in the park, 'Everything to him should have felt like a typical day, a walk in the park'. The character has no feelings, so all of his actions are done without thinking, like a robot. Nothing is easier or harder than anything else.
(englishbaby.com)

The expression walk in the park means that something is easy. It could be a walk in the park to cook your favorite recipe or even to finish a simple homework assignment. Whether it’s actually something in your free time or part of your job, anything that is a walk in the park is not a difficult task. (englishbaby.com)

(it’s) no walk in the park

....You may think that poetry class is an easy A, but, let me tell you, it’s no walk in the park. (englishbaby.com)
....I’ve been working 12 hour days all week. This job is no walk in the park. (englishbaby.com)
....I think I need to move out. I thought living with you would be easy, but it’s no walk in the park.” (englishbaby.com)
....But school was no walk in the park for me. I chose to study Physics - because it fascinates me. No, I’m not a genius. Math and science are not second nature to me like they were for a good chunk of people in my program. I had to work hard. Many Friday nights (and other nights) were spent at the Physics building until 2 am or later working on homework assignments. Most Saturdays were spent just recovering from difficult weeks, and Sundays were homework days. (

Related vocabulary:
[as easy as pie]
[piece of cake]
[plain sailing]
kid (kid's) stuff
to run smoothly
duck soup
[без сучка, без задоринки]
[it'll be all right on the night]