Lines. Try making as many different kinds of lines with as many materials as you have. Here are a couple pages of my sketchbook where I explored line:
|I used straight brush painting, dry brush, stamping, lifting, stamping, masking, and scribbling in a stencil to make this variety of squares.|
Another way to practice Variety is to set up a grid structure, and make a different mark or image in each square. This is not intended to make a composition, only to challenge you to stretch your visual vocabulary. Here are a few examples of student work:
And finally, here are a few examples of student compositions based on the idea of variety within a grid structure:
I hope you'll spice up your life by exploring variety next time you take out your sketchbook.