*Experiment with unusual materials (make up-eye liner, eye shadow), food coloring, grind spices to make natural dyes, tea or coffee, charred wood)
*Experiment with textire (q-tips, feathers, comb, sea sponge, cheesecloth, plastic wrap, fork, bubble wrap, etc. can all make interesting tools...)
*Experiment with perspective (draw an object as realistic as you can in pencil, then try it again using only ink and brush. Try painting it in simple geometric forms, or in an unexpected color or texture. Try drawing part of an object from a close up perspective, or from an unusual angle).
*Experiment with Line - Try a 1 or 2 minute contour drawing (simple, quick line drawing), or set a timer for 1 minute, place your pencil in one hand on the drawing paper, and hold an object in the other. Turn your body away from your paper and look only at the object while you draw (don't cheat and peek at the paper). This "blind contour" exercise is great for training yourself to observe. Or, try a gesture drawing... a quick sketch of an object in motion.
Let go of a desire to achieve perfection and just have fun. It's amazing how a few exercises like these will get those creative juices flowing again!