Creative Ukulele Making is a great PBL / Cross Curricular / STEAM Project that works great for all age groups.
MusicÂ
Musical notes / notation
Locating the notes are on different frets and strings
How notes make up chords
Strumming / Different Rhythm Patterns
Learn how to tune / pitch / developing the ear
How to string up an instrument
How to play simple songs
How to play along with others(jam/collaboration/different parts)
Composing
Learning major/minor scales