Is there anyway to show an advertisement banner after the first post or topic but only in ONE subforum?  I want to show an ad banner just in my classified forum section not in any other forums!  I can't see a way to do it with the built in advertisement system??

There isnt a way in which to do this on the built in system, unfortunately. You would need custom ad code to suppress it on other forums

I don't see a way to add an empty sidebox that I could drop some advertisement code or an image into?  Didn't there used to be an HTML box available in the Manage Blocks menu?

This can be done in the admin CP with the pages application. If you dont currently have the pages application, the only way from there would be to create a plugin/application or purchase pages so you can code it in there

This can also be done with javascript (without the need for a plugin or application). 

 

This can also be done with javascript (without the need for a plugin or application). 

Of course thats another way 🙂