What stuff do you have right now?
Try finding some time periods that you have the least stuff; if your lacking in Baroque (alot of people play a two part invention for a baroque piece for an exam, and don't like playing any more than that), look at Bach's Sinfonias, or if you are at the level to play Preludes and Fugues.
A few shorter works from a set is good to get some varying repertoire, rather than tackling a 15 minute piece. A few Rachmaninoff Preludes, Debussy Preludes, Chopin Waltzes, Beethoven Bagatelles, Scarlatti Sonatas, etc etc.