I have used dozens of Powell-Graphics.com availability calendars over the years and was very happy with them. They are the most attractive calendars that I have ever found and they are the easiest to install.

They design a calendar page that matches your web site and host the page. All you have to do is put a link to the calendar web page on your site to get it installed. You can easily add it in less than 5 minutes. No special code to worry about.

Their calendars are not free though. They charge $20 per year, but is worth every penny. Your client will be very pleased with the appearance and ease of use.