Why dont you do Beethovens op. 78? Its only about 12 minuets long (both movements)and its a really cool piece. It wil contrast nicely with the Rachmaninoff. And since its so short you will probaly be allowed to play the whole thing.
Schubert would be good, the 3rd movement of the A major op 120 always sounds impressive and is underratedly difficult (everyone seems to think this is Schubert's easiest sonata, I would disagree. While it is no match for the final 3 sonatas this third movement is a real pill to play correctly and well, with some wicked jumps in the left hand and some amazing juxtapositions of tonality it can be a real pain if you pick it up @ level).Good luck!!!dmk