One important property of light is that it travels in a straight line.

Equipment list: A sunny day and objects of different lengths, including people.  Choose a tall object that you cannot measure directly, like a flagpole or lamppost.  It's a good idea for the shadow cast by this 'object of unknown height' to be cast on flat ground or sidewalk - avoid slopes or steps.  Try to include at least one object that you can easily measure that is taller than a person - such as a long stick or branch held vertically to produce a shadow.  Also useful: drawing materials (pencils, paper, rulers), and string.

Activity: Measure the heights of the various objects as they are standing vertically on the ground.  Also measure the lengths of their shadows, and the length of the shadow of the very tall object (flagpole or whatever).

Scientific background: The ratio of height of object to length of shadow should be the same for all vertical objects.  This might become apparent if the students make bar graphs, with the height of the object and length of its shadow represented by adjacent bars.

To understand the reason for the proportionality, it might be useful to cut a length of string equal to the length of the shadow for each object, and another string from top of object to end of shadow (representing the path of the light); together with the object itself, these form a right triangle.  All the triangles for all the objects have the same shape.

Problems: Since the sun angle changes by 15 degrees per hour, the measurements should be made within a short period of time -- perhaps within 20 minutes.  Measuring the flagpole shadow twice (at the beginning and the end of the measurement period) will point this out, and help establish how important this effect is.
 
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