The most unchallenging piece ever

[Nightscape]

What is the easiest piece you can think of?

[Ludwig Van Rachabji]

Possibly the easiest, AND the hardest piece at the same time -

4'33" by John Cage

[abe]

The second "piano" part of a very simple duet involves clapping...although that does involve maintaining tempo and rythm.

[lostinidlewonder]

Czerny's Opus 599 First Instructor for piano. Goes from 1 through to 100 where of course 1 is much easier than 100. Goes through very preliminary keyboard co-ordination, for instance 1 is held semi breves against minum then minims in Bh in similar motion. 2 introduces contratry motion as well as maintains similar motion, using the same two note elements, minums and semibreves. He gradually introduces intervals as well, different note values, chords, tempo, dynamics, etc as you go along.
It is usually sectioned up as
1-10: First excerises for the knowledge of notes.
11-18: Excersises for the 5 fingers with the quiet hand.
19-26: First excercises for the thumb.
27-31: Excersises exceeding an octave.
32-35: Excersises with the bass clef.
36-38: Excercises in sharps and flats.
39-42: Excercises in other easy tunes.
43-57: Rests
58-70: Excercises in Velocity (which is further developed in his School of Velocity)
71-100: Melodies with and without Ornaments

[Goldberg]

Bartok's very first (and many of the earliest ones as well) Mikrokosmo...or however you say it..

[Glyptodont]

Chopsticks

[thierry13]

Possibly the easiest, AND the hardest piece at the same time -

4'33" by John Cage

Why the hardest?

[c18cont]

"why the hardest..."  (Cage)

Boredom..

John Cage

[c18cont]

Jus Kiddin

John Cont