Noodling with melodies and music-notation software. I have a ways to go with both.
In the meantime, back to Sunday in the Park With George. One of Stephen Sondheim's basic principles is that Form follows Content. (I hear you, Black Mountain poets: form is an extension of content.) This applies to his melodies. Instead of composing a tune and chord structure and verse/chorus structure and finding the words to fit, he would find the melody to fit the words, even if chorus/verse regularity is not available. The songs are about the character, not about the composer's clever ideas.
The same is true of rhyme. SS is a stickler for pure rhyme, but that doesn't mean abab, for example. He might go aba-cdcd- and then pick up that resolving b.
That principle--Form follows Content--gives us songs in which a character develops, which allows an artist like Bernadette Peters to give everything to the moment she's playing.