Need random number in "header" template

[forum_maniac]

I need a random number in the "header" template to randomly show different type of information. I know I can do this with Javascript but since some of the things I want to display is AdSense code I cannot use Javascript as I would have to slightly modify the AdSense code (which Google does not allow).

So I want to use a hook to define a variable and assign a random number between X and y to it. Then in the "header" template I use an <if> statement to evaluate the variable.

Questions:
- What hook should I use?
- What is the syntax of the variable in the hook?
- What is the syntax of the variable in the template?

You help is appreaciated.