When the US occupied Japan, it was unlike the US-led occupation of Iraq or other occupations where there was a hostile populace under subjugation by the occupying forces and a lack of a compliant regime in place to give legitamicy to the authority of the occupation forces. The US had been planning a comprehensive post-war program for Japan for some time and had engaged the cooperation of Emperor Hirohito, who was allowed to remain in his position as nominal leader and moral and spiritual figurehead to the Japanese people. The emperor's representatives agreed to the terms of the formal surrender on September 2, 1945, in which the emperor and the Japanese government agreed to recognize and submit to the authority of the Supreme Commander of the Allied Powers (General MacArthur). The terms of the surrender and postwar program for Japan had basically been laid out in several statements from the allies in the months leading up to the capitulation of the Japanese govt (Postsdam Declaration, Byrnes Notes, etc). Secondly, the military leadership of Japan called on their forces to lay down their arms and requested cooperation and adherence to the surrender by all personnel.

So, the Japanese people were not under pressure from internal or external groups to throw off the occupation, as is the case in many hostile take-overs. In addition, the infrastructure of the actual government and the throne of Hirohito were kept in place (of course, many responsible for the initiation and conduct of hostilities were removed and punished). Remember that the Japanese had been living under servere auserity and an environment where extreme sacrifice was required to carry out the Japanese military's campaigns since the early 1930's (not to mention that most major cities had been decimated by American bombing raids and their economy was in a shambles, millions were starving and homeless). The allies waved the carrot of a large number of progressive reforms that would benefit the individual Japanese, as well as give hope of ending the feudal and often repressive system in place.

Even though the occupation was exremely humiliating to the Japanese peole, they basically pitched into rebuilding their country along the lines of the democratic reforms laid out by the allied command as they had in supporting the military ventures in prior years. Based on my readings, the American occupiers were seen as somewhat benevolent benefactors (especially MacArthur who was venerated by many Japanese) who were helping in the rebuilding of their country and implementing much needed reforms.

I know there were some die-hards and radicals who did not agree with surrendering, but overall most Japanese were exahausted by the war and relieved that it and all it's hardships were over and were willing to cooperate with the American occupiers.

So to answer your question, there would probably be little danger to a lone American venturing out to legitamite businesses like clubs, bars, and restaurants. Of course, there were some places that any person should avoid (shady areas and gang-controlled sections like any city would have), but in general it was pretty safe for Americans in post-war Japan if they didn't stray too far from the beaten path and kept their noses clean.

To learn more about this and to cite the books I took most of the above observations from, I would recommend these works of John Dower (or any of his works on Japan):

Dower, John W. (1999). Embracing Defeat: Japan in the Wake of World War II. W. W. Norton & Company. ISBN 0-393-04686-9. Winner of the National Book Award, John K. Fairbank Prize of the American Historical Association, and Pulitzer Prize

Dower, John W. (2010) Cultures of War: Pearl Harbor, Hiroshima, 9-11, Iraq (New York: Norton : New Press, ISBN 978-0-393-06150-5).