zdeb, my first thought was Google Analytics. I haven't personally used it, but I've seen some of the stuff @clong has done with it and I think it would probably work for what you're wanting. Here's a post with the link to more information - Google Analytics for Course Activity and FERPA
Hope this helps!
zdeb, since we haven't heard back from you, we're hopeful that @kona 's recommendation to use Google Analytics has helped you in your quest to collect analytics for a public course. Accordingly, we're going to mark this question as "Assumed Answered." The discussion remains open for participation, so people can continue to contribute their ideas as they arise.