I'm pretty sure it's a horehound of some kind which yes, most people would consider a weed. Does it smell vaguely minty?

Might be catnip/catmint.

Yeah that's a good call, probably more likely in a garden. Though many Lamiaceae look sort of like this. Could even be a plectranthus seedling but obviously that would not be so far along in zn 6.
At any rate, a lot of people consider catnips weeds, unless desired! Mints, catnips, even Monardas, they all spread like crazy if they are happy.

I don't think it's catmint or horehound since it doesn't really have a minty scent. It smells like a green leafy vegetable. Also, the leaves are very large. I know it's hard to tell scale in that photo, but the bottom leaves are almost 10" long.

Check the stem. If it's square, it belongs somewhere in the mint family.

Salvia ?