Grid Drawing — Great practice on contour lines, shading, value, etc. with easy to learn grid drawings. 5 of each, they get progressively more difficult. Grid drawing is great for:
It allows you to produce an accurate line drawing by reducing your subject to a series of small squares
It can be used as a tool for composition and design
It gives you a reference point to work from
It helps simplify complex subjects.
It improves accurac
 
Use worksheets for sub plan, intro lesson, bell ringer, warm up (my favorite early on in drawing unit) etc.