I recently joined a contest in sketchoholic. The challenge is that there are given three topics/subjects/object etc. to draw. One has to be drawn in 15 min, one in 30 min and the last one in an hour. The hour long image is usually a more elaborate one. below is my entry.
orchid flower (15 min)
lazy bird (30 min)
robot with unusual friend (1 hr)
I had so much fun in this contest. It's a very good way to practice drawing and crunching your creative brain to work in a limited time.