Switching off/on sideboxes on user site

Been on this for a while now, and seem to have a bigger problem then I thought to switch off a sidebox.

To explain my problem: I want to hide the categories side box in a situation where I want it replaced with another sidebox. The button for these two sideboxes, is inside the header menu, and I want them to toggle the sidebox on/off, meaning to switch between both.
This is all done on the 'user' side, has nothing to do with admin editing or even installing sideboxes, full control about the sideboxes goes to the 'user' side, not the admin section.

Any thoughts? I know I can switch off the categories using css but somehow it doesn't feel right to make a huge .css visible/invisible switch and have the contents of two sideboxes placed behind it...
I hope I make myself understandable here. Finding this topic on zencart forum is difficult, everything is about admin controlled or installed sideboxes, and that is not what I want.
User side "Dynamic behaviour" is what I am looking for.

Any comments appreciated,

There's a template (can't provide a link right now) that allows visitor control over sidebox display, but I'm stumped as to why you'd want to do that.

Boy, this feels like 'slow' MSN :)

Thanks for the answer, I have a catalog sidebox which I want replaced with a live chat box, so people don't have to scroll down and up etc.
An example (should be in JS i think, but PHP is fine as well), would be appreciated.

OH, and it is just for two sideboxes, not like a visitor gets to bump sideboxes all over the place :)