Really? I didn't find the Nocturne all that hard (and btw, it is one of the Lyric Pieces)
It's harder than many of the other Lyric Pieces, but according to the Pianostreet music, it's Grade 5. It's by no means the hardest Lyric Piece, so I strongly recommend it too.
If you want a truly harder one, try Wedding Day at Troldhaugen, Op.65 No.6.
Some other really good ones, varying in difficulty:
Arietta, Op.12 No.1
Norwegian Melody Op.12 No.6
Berceuse, Op.38 No.1
Elegy, Op.38 No.6
Canon Op.38 No.8
Butterfly, Op.43 No.1
To the Spring, Op.43 No.6
Norwegian March, Op.54 No.2
Vanished Days Op.57 No.1
She Dances, Op.57 No.5
Sylph Op.62 No.1
Brooklet Op.62 No.4
Ballad Op.65 No.5
Once Upon A Time, Op.71 No.1
Peace of the Woods, Op.71 No.4
Remembrances, Op.71 No.6
That should get you started.

Phil