Do you actively take pictures of the people who visit your winery? You should, and here's why. People like to share (good) pictures of themselves, particularly if those pictures offer some sort of "bragging" opportunity ("Here I am in Wine Country!").
Assign someone to take those pictures, and post them on your Web site (ideally with each group on a page of its own). Make sure when you take the picture to get the e-mail address of at least one person in it. Then, when the photo is posted, drop them an e-mail with the URL of their picture. They are almost sure to share it with their friends, bringing new traffic to your site.
(Update: my assumption is that you will ask before taking anyone's picture. Don't want to creep anyone out. I think that most people will appreciate a group shot with everyone in the picture.)
Unfortunately, some wineries have Web sites which require an act of God to change/update. That's one of the reasons that blogs and Facebook Fan pages have become popular winery tools - they're designed for easy updates.
If you do use blogging software, your e-mail notice should invite your contact to leave a comment on their visit and identify the people in the picture. On the page with the photo(s), you can add a link to a "special deal" for those visitors, and/or a link to information on visiting your winery. At the very least, make sure that there's an obvious link back to your Web site. Remember, the goal is to get new people interested in your wines (and maybe even buy some).
While you can use your Facebook Fan page for this purpose, you'll have more control over the form and content (and better measurement) with a blog page or a page on your regular Web site.
It's probably be worth paying some aspiring college-age blog-savvy photography student to do this as an experiment, perhaps on a weekend when you're expecting lots of visitors. If you're running Google Analytics or otherwise tracking your visitor traffic, you should be able to quantify the results. Be advised that this sort of traffic-building effort takes time, however.
Of course, it all goes back to being able to add content to your Web site easily. Can you?