There were some discussions by scientists and the Secretary of War of whether a non- or less-lethal "demonstration" could have been done instead of dropping them on cities, yes. It was explicitly rejected, and no mention of it was made to Truman. See this article on the "demonstration" question.

Separately: I find it plausible, in my own research, that Truman actually thought he made a distinction between dropping the first bomb on a city and a military base, but he misunderstood that Hiroshima was a city with a military base in it, and not "purely" a military base. You can see some of my preliminary research on this here and here. A peer-reviewed article is coming out on this next year.