**Use a Rock:**
(For quick results skip to NOTE)

I had one that was more for fun than anything else. You can use a rock (Assuming that the height of the tree is within throwing distance, you can choose a small rock) and use the 1 dimensional kinematic equations from classical mechanics.

That might sound kind of scary, but really just throw the rock as hard as you can and start counting seconds, and when you see it reach the top of the tree, and record this time.

You can then use the equations:

Where delta x is the height of the tree, v0 is initial velocity, vf is final velocity (Which is 0 since it stops at the height of the tree, or approximately the height of the tree), and t is the time you measured to get to the top, which you have from counting. You have two equations and two unknowns (delta x and v0), you can solve this by substitution, elimination, or by matrix algebra (RREF) to get the height of the tree). NOTE: the height of the tree will be in meters, you know, the better format :D

NOTE: That's the complicated way, the simple way, after derivations is reduced to:

We see that the result is easy to compute in your head, it's:
So, if the rock takes 2 seconds to stop in the air, it went a height of approximately: 5 * 2^2 = 20m