Tags are as wide open an area to explore as they come. You can make up a tag on the spot for just about anything. The more niche, though, the less likely anyone will go looking for content using that tag.

A couple Nostr clients show you what tags are currently popular. Ditto.pub, for instance. If you go to Primal's explore tab, and then over to the Topics tab within that, it's basically a list of currently popular hashtags as well.