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.
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.
Find 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.
Practice drawing typography.
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.
Make many quick sketches of people in dynamic poses.
Look at reference, put it away and draw from memory. Use reference to fix mistakes, and then do it again.