Action/Adventure is often considered Men’s Fiction, like Romance is Women’s Fiction, but women like good a/a, with either gender of protagonist, just like some men like to read romances (note: the same person often enjoys both).
Action/Adventure emphasizes just that. Personal relationships are strictly optional, though men’s fiction a/a often has the sexy babe who is kind of the hero’s reward at the end, or some along the way to create rescue objects, dead ladies to avenge, or heartless Delilahs who only use the hero in order to betray him. Okay, I’m assuming all sorts of gender things. Today we all know the he’s and she’s can assort all kinds of ways. Let me stick to the traditional and, still, marketing majority of hetero male a/a, or it’ll take five times as many screens to talk about this.
By default, a/a takes place in urban or suburban places unless otherwise marked. Think Bourne. Think James Bond. There can be elements of suspense, thriller, or mysteries here, but the reader is waiting for the explosion into violence, the honed ability to rely on oneself to get through, and even save others. It’s a matter of physical as well as mental or emotional adventure.
So, as the examples tell you, this is the place for your espionage fiction.
There are some settings that are so well-used that they form subsets. These can be contemporary or historical. Remember, in fifty years your contemporary will be historical, for all purposes. Make it rely on more than the gadgets (that will be so old then) or just accept that it’s going to have a short shelf life. As well, watch your world-presentation.
Outdoor adventure moves your hero into places where people are few and far between. It may be Man vs. Nature, or he may be pursued or pursuer.
Contemporary military fiction features the modern soldiers and pilots on various missions, overt or covert. Their being military counts in the plot, as opposed to just outfitting some other sort of tough guys. Historical military fiction might be typified by the Sharps series, of an English officer in the Napoleonic Wars.
Sea Stories have a wide range of subjects, from stories about racing yachts to the equivalent of wet outdoor adventures, to naval fiction. The contemporary naval fiction takes place usually in smaller vessels: it’s hard to feel significant as one of a crew of 1200. Historical naval fiction might include WWII or even WWI, but is especially popular for the Age of Sail, whether it’s the classic Horatio Hornblower series or the more modern Aubrey and Maturin stories. Both are set during the Napoleonic Wars, but Hornblower stays closer to home, I notice, rather than sailing for the Pacific.