What are you using for your scripting?  It is quite easy to create a random number generator in just about any scripting language.

If you drop some PHP code into your page, within <?php ... ?> delimiters, you can use its random number generator to pick one of a number of pre-built chunks of text and HTML code (page segments) kept in separate files. I presume your site is currently HTML; you would either have to specify that .html/.htm files are parsed as PHP, or change your file extension to .php. To be honest, static HTML pages like you probably have are very limited. You'll want to get into dynamic pages built on the server with a language like PHP anyway.

If you want to use Javascript's random number generator, you'd have to package all the alternate content into the page (as, say, array elements) and then pick which one to use on the fly at the browser. If each alternate content is only a few words, this wouldn't be too bad, but if it's a good sized chunk of HTML text and code (or there are a lot of them), it would really bloat your page size and slow down loading.