I don't know exactly what you mean by early intermediate, but the easiest Rachmaninoff piece I know of is his 'Morceau de Fantasie' in G minor. It was written when he was very young, and it's not very well known, but it's a very nice "first rachmaninoff piece".(this is one of the only versions I found on youtube)If you really want to play his preludes, try out his famous prelude in C sharp minor. I found it much "easier" than most of the others (It's still not that easy, I don't know your level so you just have to try it out yourself).Good luck!
Piulento, Do you know where I could find the sheet music for this?
Maybe if you wanna play Rachmaninov, you could try with one of his etudes tableaux, for example op. 33 no 7/8 in G minor? Not difficult, yet so satisfying.