Create custom post type that is then available as a WYSIWYG Editor option for links to be opened in modal window.
Sometimes content doesn't need it's own separate page but it's enough that you don't want to jam it all into one page. Modals are a great way to handle situations just like that. This creates a new post type and menu option in the main menu in the admin site. Add posts here just like you would in Posts or Pages. Then go into the Post or Page (or other post type you may have) and there is a new option in the editor toolbar. This allows you to create a link that will open one of those Modal post types into a modal window rather than sending the user to a separate page.