If it's your own instance, make a bot account who can follow a few interesting accounts from other instances. That way the local timeline gets filled a bit, and it's not so lonely.

A few things around this, to gain traction, a person needs to have 5-6 accounts to interract with regularly, before they catch the addiction. But if those accounts are active, they'll quickly get out there, into a big bad world.

Depending on their age, have regular sessions with them. Go pick up an interest - a hashtag, a specific prolific creators' timeline, something like that. If you spend a bit of effort, this is easy to schedule. Point out the bad actors, point out the funny memes. Ask them how'd they respond. Teach them what a social network can be, and what they can do there.