Outside of clicking on the notes, you can make use of the QWERTY keyboard. 2, 3, 5, 6, 7, 9 and 0 can be used [not the keypad] along with the letters Q, W, E, R, T, Y, U, I, O, P, S, D, G, H, J, Z, X, C, V, B, N and M. The letters and numbers are split between the black and white keys
It's nice to play around with especially if you're looking for a keyboard simulator to practice on. I'm not musically trained but have a somewhat decent ear. Compared to others I've experimented with this is more user friendly as you can go by what you hear as opposed to the actual notes.
Do play around with the duty cycles as they each have a unique sound and feel.