Leaving aside for a moment arguments about Hiroshima's own status as a military target, was there any discussion by American commanders of dropping the bomb on a more obviously military target in 1945? Like Japanese positions on Mindanao? Or in China?
I don't think you can leave aside arguments about Hiroshima's status as a military target. Hiroshima was the HQ of the Japanese army responsible for defending all of southern Japan. The Field Marshall given this command, Shunroku Hata was actually in Hiroshima during the attack (he survived and supported the Emperor's decision to surrender). Looked at in light of the upcoming invasion Hiroshima was as high value a target in purely military terms as existed in Japan in 1945.
The first real target suggested for the bomb, as early as 1943, was for the Japanese harbor in Truk Lagoon. This was sort of the Japanese equivalent of Pearl Harbor, and was a pretty distinctly military target.
By the time of 1945, though, the idea was that the bomb was to be a spectacle, and nothing would showcase its power more than destroying an entire city. By the spring of 1945 it was already the assumption amongst the bomb designers and targeters that the first uses would be against built-up cities with industrial and military facilities embedded within them.
The idea that future bombs might be used for "tactical" purposes — e.g. to aid in an invasion landing — was discussed explicitly in the period between the use of the bombs and surrender.