Being able to look at a whole note, or a half note, quarter and eighth and really truly understand how they're all related and how they fit into a time signature. 5th graders should know how to remove a chunk of ten 16th notes from a 4/4 measure full of them and what combination of half, quarter and eights they can replace them with to still end up with a complete measure.

Grand Staff exposure. They should not only know the acronyms for each staff, but should see how they're related, how Middle C connects the two and how Treble Clef notes are higher than Bass Clef notes.

Solfege integrated with singing. Your MS and HS choir directors will love you for this one, but even we benefit from this because it does wonders for ear training.