My very favourite piece of music for the piano is Wedding-Day at Troldhaugen (if I am ever engaged to a woman who refuses to have that piece played at some point in the wedding or reception, I'm calling the wedding off), and Grieg is most famous for the Peer Gynt suite (perhaps more cynically, specifically for Morning Mood and In the Hall of the Mountain King), but there's so much more out there. At this very moment I am listening to Twelve Songs, Opus 33. Varen (Last Spring), but really you could listen to almost anything from his lyric pieces or Norwegian Peasant Dances and just feel content with life.
Worthy of special mention: Arietta, the Holberg suite, Haugelat (Tune from the Fairy Hill), Du Fatter ei Bolvergenes evige Gang (The Poet's Heart), and probably a whole load more which I have yet to discover.
Unfortunately my book of Grieg pieces is currently at home in Birmingham - my brother is actually learning pieces from it - but I've asked for a new copy for Christmas. I'm now actually slightly hoping not to receive a copy, just so that I have an excuse to buy myself an even bigger book of Grieg pieces.