What about Chopin's Nocturne in G minor, Op 15, No 3... its graded at RCM grade 8 (so according to RCM, its the 'easiest' nocturne), and its BEAUTIFUL. (I haven't learned it though, but if I remember correctly it doesn't have crazy polyrhythms like most of his other nocturnes do... )
For Beethoven, Rondo in C Major, Op. 51, No. 1 is nice.
If you can play Claire de Lune, maybe you could try Prelude in C# minor by Rachmaninoff, I think they are around the same difficulty. I really like Chopin's Waltz in A flat 69 no 1 as well. You didn't mention Bach, but personally I found his little preludes a lot of fun, and they definitely help build skills.

I like the one in D major, bwv 925. They are probably considered less complex than the other pieces I listed, but I don't know. To me, any piece I cannot play is difficult!