What I'd like to happen, when all is said and done, is that the user would be able to drag and drop notes (of varying lengths, each represented by a separate item) from the palette onto the music sheet. Then, a dialog box would pop up asking the user to fill in some details (such as the octave, key, and instrument), and then the note would be dropped. (I'd like the dropped buttons to be better than they are right now, so that the sides of the button wouldn't be visible and the note would be the only visible part of it.) The Options window would be used for setting defaults (such as the default instrument, so that the user wouldn't have to select an instrument per note, unless wanting otherwise). As the user drops an item, the sound would be heard (thanks to the JFugue API). If the user were to want to, the note could be dragged somewhere else. Whenever the user wanted to, a "Play All" button would be clicked. And there'd of course be the possibility of saving the current music string (generated by the dropping of the notes) as a Midi file.
Further enhancements (such as different voices and more complex functionality provided by the JFugue API) could be added afterwards. But the scenario in the above paragraph is basically what I'm shooting for.