In case you missed it, there's a section of Mastodon which lets you discover trending posts, trending hashtags and trending links.

📱 On the official app, click the magnifying glass 🔍 at the bottom and don't type anything into the search box. By default, you will be shown all the latest trends as Posts, Hashtags, News. (You'll also see the latest posts by people on your server in a tab called "Community").

:mycomputer: On the website, click the # Explore link on the right side of the screen.