Shikantaza

Shikantaza is usually translated as "just sitting" or "nothing but sitting". It is the central practice of the Soto school of Zen and is closely associated with the thirteenth-century teacher Dogen.

Unlike breath counting or koan work, shikantaza gives the mind no particular object. You sit upright, fully awake, aware of whatever arises, and neither follow thoughts nor push them away.

It sounds like the easiest practice and is often called the hardest, because there is nothing to hold on to. Many people start with counting the breath and come to just sitting later.