One of the things I try to do when given a sketching assignment is try to think of objects that are not typical solutions. In sketching "light coming through water/bottles", I choose this water cooler because I thought it added variety amongst sketches of regular plastic water bottles. This sketch was one of my quickest, at about 15 minutes, and was done in my home.
An upside down wine glass shows captures light reflection and, again, adds some variety. I spent 20-30 minutes on this sketch at home.
One of the individual pieces of the modular 2x4 shelving system. I spent about 15 minutes on this sketch, and really think it was one of my more successful "quick" sketches, because of it's layout and simplicity.
Everyone groaned when the assignment for sketching self-portraits was given, myself included. It's been about 7 years since I even attempted doing a self portrait, so for my first sketch I used a photograph and the grid system. This one took about an 1 1/2 hours to complete.