You have one octave to play with. The notes are as follows.
Note
Button
PC
PlayStation
Do
[Cancel]
[0/Ins]
Cross
Re
[Switch]
[Decimal/Del]
Square
Mi
[Menu]
[+]
Triangle
Fa
[OK]
[Enter]
Circle
So
*[Cancel]
[9]+[0/Ins]
L1 + Cross
La
*[Switch]
[9]+[Decimal/Del]
L1 + Square
Ti
*[Menu]
[9]+[+]
L1+ Triangle
Do'
*[OK]
[9]+[Enter]
L1 + Circle
Asterisked entries mean you need to press either [PgUp] or [PgDn] while pressing the button in question. Therefore, either [9] or [3] on PC or L1 or L2 on PlayStation work.
You can also play three different chords.
C
Down
F
Left
G
Up
C'
Right
If you play all the notes in order (as in on the list that comes up, you receve tifa's secret savings, a mighty 1 gil.
Playing piano is ended by pressing [Start] ([5] on PC, Start on PlayStation).