The biggest problem with a "work there for a day" interview IMO is that there's no time to do anything useful. You can't get familiar with a codebase of any size in a day.
Welcome to pair programming. Unlike real pair programming, don't let the noob touch the keyboard for the entire day. Talk talk talk. Don't expect either to really get much work done. But pretending to work all day is a heck of an interview.
This works outside of programming. Over a decade ago when I was doing network stuff, I did "pair router work" with a couple people. You don't give a noob enable CLI on a production router on his first day, or during an interview, either. Even if on paper, or in reality, he is better at this than you are.
If you hire them, you'll be spending the first couple days like this, anyway, unless you go all sink or swim.
This works outside of programming. Over a decade ago when I was doing network stuff, I did "pair router work" with a couple people. You don't give a noob enable CLI on a production router on his first day, or during an interview, either. Even if on paper, or in reality, he is better at this than you are.
If you hire them, you'll be spending the first couple days like this, anyway, unless you go all sink or swim.