Would have been easy to drop one on, say, Tokyo bay if you wanted intimidation. But the US also wanted reliable data on the effects to buildings and people.
I really wouldn't be so sure. Even after destruction of Hiroshima and Nagasaki, the Japanese military still opposed surrender, in fact the day before the Emperor was supposed to announce it there was an unsuccesful coup attempt to prevent it.
So no, I don't think just detonating the bombs "harmlessly" would have have had the same effect. In fact hitting a target just slightly off the mark(like Tokyo Bay) would have been presented as American incompetence and failure of aiming.
Were the atomic bombs really all that different from the systematic fire bombing of Japanese cities?
I think what got Japan to surrender was a certain common sense in the emperor and some of the elite that a final last stand wouldn't do them much good. Especially not with the Soviets joining in.
So no, I don't think just detonating the bombs "harmlessly" would have have had the same effect. In fact hitting a target just slightly off the mark(like Tokyo Bay) would have been presented as American incompetence and failure of aiming.