virating strings. anything with a rhythmic smooth repetitiveness.
also check out applications of fourier series even though fourier series will probably be beyond the scope of the class, you can see where they are used and say that sine/cosine are used there.
sine/cosine is also used in the rotation of shapes in pictures and graphics such as 3D computer graphics which has nice formulations in 2x2 or 3x3 matricies from linear algebra.
you can also mention how sine/cosine are used to prove what some think of as one of the most beautiful equations ever (attributed to euler):
from which one can conclude
[tex]\ln (-1)=\pi i[/tex]