Find something you think looks beautiful - a human, location, object - and draw it carefully from observation.
Practice drawing organic volumes. Draw cross-contour lines to to make sure you understand their volumes.
Everything can be approximated with boxes, spheres, cylinders and cones, and it is a good exercise to practice drawing these from many orientations.
Look at things, and find the abstract two-dimensional shapes in them.
Draw one-inch rectangles, and try to come up with creative textures to fill them with.
Various exercises that help you become better at drawing lines accurately.
Create a sketch with a strong focal point.
Draw something with one single line.
Typography is a fundamental of visual design. Practice drawing type!
Practice drawing folds.
Practice drawing many small quick thumbnails to design something.
Practice drawing animals.
Practice drawing hands.
Draw cartoon or comic variations of faces.
Practice drawing the various facial features.
Try out different ways to construct the head in many orientations.
A key drawing exercise to study human poses, and a great way to warm up and create a beautiful sketchbook page.
Drawing from memory is the best drawing exercise I know. If you choose one exercise to become better at drawing, choose this one.
Find reference of a pose, and reference of a dressed person, and pose that person in that pose: combine two reference photos and turn it into something new.
Take a stain or an object and turn it into something else you can see in it.