If you don't want to have to create an index file in each directory then htaccess is really your best (and perhaps only) option.
You can either set a redirect/rewrite to a page that does exist (which, from your question, I assume you know how to do already) or you can specify your default index page so that users can still visit /folder/ and they will see your content.html page.
Setting the default index page to be your contents.html file would look something like this:
Code:
DirectoryIndex content.html