This describes how to determine the height of a tree with only a stick (or trekking pole). It could be useful in selecting a safe campsite, or just for fun if you see a really tall tree.

From http://www.americanforests.org/resources/bigtrees/measure.php

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Hold the stick at its base vertically, making certain that the length of the stick above your hand equals the distance from your hand to your eye.

Staying on ground level (or on the same contour as the base of the tree), move away from the tree while sighting the trunk base above your hand.

Stop when the top of the stick is level with the top of the tree. You should be looking over your hand at the base of the tree and, moving only your eyes, looking over the top of your stick at the top of your tree.

Measure how far you are from the tree and that measurement - in feet - is the tree's height.
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The Boy Scouts have been using this method for a hundred years. Since we are talking about “estimates” here, the Boy Scouts also have a method for this that is a little bit simpler, and just as accurate. It makes teaching young scouts easier and they have less to remember about the length of their arm and the lenght of the stick they use. It does, however, require that you have another person to assist.

Have your buddy stand in the vicinity of the object to be measured (tree, building, etc).
Using a stick or other straight object (ruler, pencil, pen, tent stake);
Hold your stick vertically at arm’s length.
Align the top of the stick with the top of the object to be measured (tree, building, etc).
Adjust your grasp on the stick so that your thumb aligns with the base of the object (tree, building, etc.).
While keeping your thumb at the base of the object, rotate your wrist so that the stick is now horizontal with the ground. It makes no difference if you rotate left or right.
Have your buddy move to the point where he/she is aligned with the end of your stick. Mark the location with a rock or other object.
Measure the distance between this mark and the base of the object and you have the height of the object.

Could just get a clinometer. :)