From the baroque era I suggest you to try scarlatti sonata L 118. It's interesting to play and for your level ( at 18 years) it's easy.Rachmaninoff is another challenging composer. I advice you to play Etude Tableaux Op.39 no.2. It is easy to play but it's challenging in phrasing. Another piece by Rach you can do in a short time is the prelude op.3 no.2 or elegie op.3 no.1. I play the elegie and I am 8 grade but it can be played even in the 12 grade ( depends on how you understand the piece).A sonata by Beethoven ( only the first movement ) or papillons by Schumann Op.2 no.1.I hope that this will help you!