When there's nowhere safe/boring enough for a naughty step. I end up putting him in his cot but don't want him to associate his cot with punishment really. Plus he's quite happy there after a bit so it's not really punishment! He's nearly 2.5yrs and pushing the boundaries!!!!
We put our almost 2.5yo DD in the hallway. It is central enough that we can see (and hear!) her, but boring enough that it is a discipline. It's more for a time out anyway, the (pretending) to ignore her bit is more of a consequence.
What's the particular issue you have - is there something he can reach from everywhere?
I would maybe put a chair facing the wall or in the middle of the room. I have on occasion just put DD sitting in the corner of the room. But I think the key is to try and find that one place in the house and keep using it so it reinforces the concept of time out. You could maybe buy a small mat that can be put in the middle of the room and he has to sit on that?