I just found out my teacher has some insane obsession with the raincrap prelude so I guess I'm playing 1, 15, and 16. And I still don't know about that "other piece" but I need to fill 25 minutes and I only have
Bach - 3 1/2
Ginastera - 3
Chopin preludes - 7
so I need something very long I guess. 
Isn't it OBVIOUS???
Don't just do 3 Chopin Preludes, do 6! JK
Or you can do an opus of Scriabin Preldes. Op. 16 is pretty, and it's only 10 minutes long!

Psh don't do Debussy. Debussy is really boring when played live. Except for l'Isle joyeuse & Jardins blah blah crap and Reflects dans l'eau.
Oh, and a sprinkling of some well-thought-out Grieg doesn't hurt: (starts at 3:25)
Or try a complete opus of the Lyric Pieces:
BTW, my favorite preludes are #1, 2, 3, 4, 6, 11, 17, 24. But that's probably completely irrelevant by now

You can do the entire Mozart K311. Without repeats, it's 12 minutes. Or Haydn's complete HobXVI32, which is NINE MINUTES!!! I think classical sonatas are very refreshing to the ear.
Clementi is nice, too, but...