I recently replied to someone on an mail art forum looking for various Drawing Exercises to keep them busy.
Here are fifteen exercises I listed in case you're stumped and want to practice drawing in your sketchbook and beyond.
1.Cut a picture out of a magazine, rip it in half. Draw the other half.
2. Draw your to do list.
3. Get a cheap mirror and practice drawing your face, or parts of it.
4. Draw your latest finds from the farmer's market or grocery store.
5. Find a new park in your area, draw 5 quick sketches.
6. Think of ten adjectives...try drawing each one.
7. Sketch your pet. Practice drawing textures like hair.
8. Draw on spare paper you may have...including junk mail. Doodle away!
9. Draw grass outside, as silly as this sounds, you're practicing using line and tones at the same time.
10. Stumped on what to draw? Try making Pencil rubbings of various objects/grounds and create an abstract composition.
11. Try to recreate a scene told from a newspaper article or news story.
12. Try experimenting with new media, I once dipped random objects found around the house in ink and tried just making new marks.
13. Do a daily drawing for a certain amount of time, many artists do this anywhere from five minutes to an hour. Pick a time that suits you best.
14. Ask a friend if you can draw them. One of my friends from graduate school needed models but didn't have anyone to sit for him so he made drawings of all his facebook friends from their profile pictures.
15. Create new patterns you haven't seen before but can imagine on a wall or rug.
Hope these are helpful to someone out there! Draw on!