5. Build a micro-website or blog
By creating a micro-website, you can grow the number of backlinks for your main site. This technique does not violate Google's guidelines; it is totally safe for your website.
Moreover, you can generate only high-quality backlinks since you can choose what anchors and links to use in the content on your micro-site. You do not need to duplicate the content there to make it unique and relevant to boost the visibility of your primary website.
A nice example of utilizing this grey hat SEO technique is the affiliate blog
where the regularly describe the practical tips relevant to their industry and naturally, put a lot of backlinks to the primary resource.