If you ask a web designer, it's easy to make a site interactive. But if you want to do it cheaply, here's one way. To make it interactive, you need to provide a way for visitors to post content. How do you do this with a static website with no server side components? Check out forumatic.com. It's a way to create your own forum on forumatic's site, which you administer, and which your viewers can join and post to, and read messages from. You can also set it up so that they don't have to join the forum to post on it and can post anonymously. Another feature is it tells you how many visitors viewed each post so you can easily measure usage. Forumatic is free as long as you don't use it too much. Also, your forum gets a name such as yourname.forumatic.com. Once you've created a forum on forumatic, you can link to it from your static website. As an example, see http://www.khitchdee.com
If you know of other cheap ways of going interactive, post on this thread.