This 6-inch Westcott ruler from C-Thru is rigid enough to toss in a back pack, but flexible enough to align around a coffee mug for measurements of curved surfaces. In our experience, it retains its straight form well even after multiple moderately curved applications. It's also a handy to tuck in your favorite sketchbook to keep around as a portable straightedge.
 
Inches are marked along the right margin with centimeters on the left.