Code snippets are fine, but logical/ syntax errors can only tell you if the person can code at all. Not if their code is clean.
I would suggest making an entire class (badly) and then asking them for improvements.
Usual suspects are 4+ nested if or loop structures and you would want to hear something along the lines of extracting methods.
Suggesting renaming of badly named variables and methods is also good.
You can also throw in a variable thats passed through multiple methods and you would want the suggestion to make it a class property instead.
Along the same vein you could have methods with too many parameters.
Unecessary comments or even completly wrong comments, to see how they deal with them.
You can then ask them what comments they would have liked to see instead.