I think it is lemon balm. If you rub it between your fingers, do you get a lemony scent?
It could be catnip but I think catnip has a kind of silvery stalk.
I have lemon balm growing in my backyard and ds3 (5) loves to pick it and rub it between his fingers to smell it.
it looks more like catnip than lemon balm to me. but then, i get these confused all the time when i have cuttings in vases. but as a pp said, you can tell if it is melissa (lemon balm) by the scent, for sure. does it smell lemony at all?