Since a lot of Rachmaninoff's works sit around amus or beyond, it is relatively hard to find many under this level of difficulty, at least some of his greater pieces anyway. Based on my own knowledge and looking at the music from his preludes I'd suggest op. 3 no. 2, maybe op. 23 no.1, no. 10, op 32. no. 7, and no. 11. You might also like to try the piano arrangement of vocalise (I prefer Zoltan Kocsis's version), or perhaps his serenade op. 3, obviously his early works do not display the level of compositional development as his later works do, and most of his technically and musically easier works happen to be written earlier on in his career.