A walk in the park is a cliché that signifies an extremely easy activity. If you don't know a walk in the park is relaxing, how on earth do these city people survive in the hectic, harried environment I keep reading about?
In other cliché-based advice to frazzled city folk, let me suggest a picnic and a piece of cake. But remember: You can't eat your cake and have it. (Which is often, confusingly, put in reverse chronological order as You can't have your cake and eat it, or as we coddled Baby Boomers like to think: "You can have your cake and eat it too.")
Which is the oldest of those 4 clichés? It's You can't eat your cake and have it, going all the way back to 1562, according to the (unlinkable) OED:
1562 J. Heywood Prov. & Epigr. (1867) 79 What man, I trow ye raue, Wolde ye bothe eate your cake, and haue your cake?A walk in the park is traced back to 1937:
1937 Amer. Speech 12 155/2 A walk in the park is their [sc. golf caddies'] facetious way of referring to a nine-hole round.A piece of cake goes back to 1936:
1936 O. Nash Primrose Path 172 Her picture's in the papers now, And life's a piece of cake.Picnic, in the colloquial figurative sense, is a little older:
1870 J. J. McCloskey Across Continent in America's Lost Plays (1940) 81 Oh, wouldn't that fellow be a picnic for me!