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Everyone seems to opeerate on the premise that we had to invade mainland Japan. It's an island, they had hardly any resources left, virtually no navy left...why couldn't we have just surrounded them and waited?

I'm of the belief that the invasion wouldn't have happened. If you read some of the original source material you'll find that the initial approval of the invasion by several of the major decision-makers (including Leahy, iirc) was conditional and only intended to allow planning and staging to move ahead. Magic indicated that the Japanese were holding back and stockpiling much of their remaining material (fuel, 6000-10,000 aircraft, etc) to be used to counter an invasion. I don't think that the senior military leadership would have given final approval in light of what they knew.

OTOH, there was a certain amount of war-weariness that was setting in with the American public. 200,000+ Asians were being killed every month on the mainland by the million or so Japanese troops still deployed there. And then there was the plight/fate of the allied POWs and internees to consider.

Even setting aside the possibility of an invasion and just looking at the effects on allied forces of a prolonged blockade, the atomic bomb comes out as the favored alternative. Throw in the effects of a prolonged blockaded on the Japanese people (who were about to experience mass-starvation) and the already solid case for using the bomb is further strengthened.