Young piano students that I've taught seem to do best with playing duets and hitting arpeggios of the underlying chords and then letting them evolve into a melody.

One fella now does a blues scale (his guitar teacher taught him that one and I really don't know what it is...maybe the same as Rick's.) He's writing his "own" blues song using one of the 12 chord blues chord progressions and throwing in stuff from the blues scale, some he's heard and just imitates, others seem quite spontaneous.