A deeper connection
As students become more advanced, it is important for them to study music theory. Understanding the basics of notation, harmony and voice leading creates a deeper connection with the music. This “partnership” with their pieces helps students improve intonation, rhythm, tempo control, memorization and can even ease performance anxiety.
Try a theory book
I have used Practical Theory Complete by Sandy Feldstein with students for many years. This series is appropriate for intermediate to advanced students. I like this book because it includes short and to-the-point text, examples and worksheets with frequent review lessons. Although it is written for piano students, it works great for string students as well.
Piano lessons and jazz
I would also recommend piano lessons and jazz improvisation. Classical musicians in particular get very locked into reading off the page. Although it is still possible to play very musically when note reading, there isn’t anything quite like improvising to form an intimate relationship with one’s music making. It is an avenue to flow.