You can say "سَهْلٌ جِدًّا" (pronouced : sahlun jiddan) (Lit. : very easy)

حاجة بسيطة

قطعة كعك, literally a "piece" of cake.

the exam was a piece of cake الاختبار كان مويا it is our dialect

As an idiom "a piece of cake" means سهل جداً like what Mr. Sleem has said . on the other hand it means قطعة من الحلوى