This contact form is fairly simple, but still safe to use. It works with a php file as the back end. It includes a dropdown, radio buttons, a checkbox and a simple CAPTCHA based on 'Regular Expressions'. If you need a more advanced form, with e.g. file upload, save to database, calculations etc, it will be beyond the capacity of this form, and I would then advise you to use Coffeecup's Web Form Builder.
The form on this page cannot be tested, because it is hosted on Coffeecup's S-drive. Php files cannot be run from there. So it means, if you want to use the form for your own website, you need to have it hosted on a different server. In the 'cheatsheet.pdf' that you can download here, there are a few more details mentioned, of which you have to be aware. In the download you also get the contact.php file necessary to make the form work.
With Site Designer version 5.0.3499 or later PHP files can now be included in the app. It is, however, still not possible to use PHP files on Coffeecup's S-drive. The form is otherwise the same as the earlier versions. The download file is a zipped project, which I'd advise you to save as a template. The 'form-cheatsheet.pdf' is included in the bundle.