In the admin > federation tab there are a couple of ways to populate your new instance with some content.
The Remote Server Scan lets you get a set of communities on a specific remote server and subscribe to them. It has options and a dry-run to see how many you would get with the settings you give it.
The Load communities from Lemmyverse data will pull the giant data set from lemmyverse.net and subscribe to a bunch of them. That one has less options, but I think you end up with ~200 possible communities with the built-in filtering.
Both use the instance admin’s account for subscribing. They are good ways to make the instance feel a bit more alive.