Ask HN: Strategies for mentoring junior developers?
I've always been a fan of assigning maintenance tasks to junior devs (or anyone who's new to an existing codebase.) These are tasks that are often lower priority so there is less time pressure to get them done right, and they give the devs the opportunity to learn the codebase. Working on existing code you don't know yet is also a lot easier than new development on code you don't know (and coding standards you don't know), so that makes it a better introduction for the new devs too.