A walk in the park
Definition 1 of 2
Pro Tip 1/3Not Literal
The phrase is metaphorical; it doesn't mean an actual park walk. 
Getting a perfect score was a walk in the park for her.
Pro Tip 2/3Non-serious Contexts
Best used in casual or informal situations, not in formal writing. 
Compared to my old job, this is a walk in the park!
Pro Tip 3/3Relative Difficulty
It might imply the task is easy because of one's abilities, not generally. 
For a seasoned chef, cooking a gourmet meal is a walk in the park.