Theory: Resistance is proportional to the length, and inversely proportional to the cross-sectional area, of a conductor.
Description: The three strips of conductive paper are all the same length, but have widths in the ratio 1:2:4. You can show that resistance depends on length by measuring the resistance between various points on one of the strips, and that resistance depends on area (or width, at least, in this case) by comparing the resistance measured between two points the same distance apart on each of the three strips.
Setup: 3 minutes.
References: any standard freshman physics text.