I would say 'code ownership' would be a perk. If you give them something (even small) and make them the go-to person for that corner of your codebase it will give them confidence and help them feel like they are a part of it.
I often hop into existing codebases, and sometimes I'm the only active dev on my part of the project, but many times I don't feel free to make changes, like I don't 'own' it.
That would be something really easy you could do that might help them feel important and needed, and it's pretty cheap (if they make a mistake it takes a bit of dev time to clean up, otherwise it's free).