How about shapes, like circles, triangles, rectangles etc.? In my photography class today the instructor showed us some slides of shapes he had photographed. Some were man-made items, others were more natural.
The idea was to learn to see shapes in places you'd never thought of looking for them before.
He then sent us outside for 20 minutes to look for specific shapes around the campus. Here's an example of what I did for triangles (not good quality, just an example). How many triangles do you see?
Picnic Table:
http://www.flickr.com/photos/pnwguy/356488969/
Triangles from Picnic Table
http://www.flickr.com/photos/pnwguy/356488923/