If the Japanese had surrendered after only one bombing, I think it would have been possible to wonder if the bombing were truly necessary. But as they chose to go on after the first bomb, had we not used the second, what on earth WOULD have stopped them??
Like I said, these things could be debated endlessly (and they are, on FR, starting usually 2 or 3 August and until the end of the month, every year, without fail!) :-)
I suppose the option would have been either to produce more A-bombs and hit Nagoya, Kobe, Okayama, Osaka, Sendai, whatever, or a full scale ground invasion in September of 1945. As it was, the US military arriving by air and sea in 1945 were confronted with a former enemy, laying down weapons, rather than a current enemy, hitting them with weapons.