I would probably say that the men at the beginning of the story are talking about a few specific things. The first thing that they are talking about is their overall poor health. The narrator tells readers in the second paragraph that they were all feeling "seedy." He then goes on to explain that he seems to have every symptom of whatever particular ailment he happens to be reading about. It becomes clear to readers fairly early on that the narrator is a hypochondriac.
Once the men's "poor health" has been discussed, the men decide that their various ailments have all been brought on by being overworked. They believe that they need rest more than anything else.
What it was that was actually the matter with us, we none of us could be sure of; but the unanimous opinion was that it—whatever it was— had been brought on by overwork.
“What we want is rest,” said Harris.
“Rest and a complete change,” said George. “The overstrain upon our brains has produced a general depression throughout the system. Change of scene, and absence of the necessity for thought, will restore the mental equilibrium.”
The conversation then moves to discussing the method of obtaining said rest. Several ideas are brought forth, discussed, and shot down. Eventually, the men decide that a camping trip along the river will fit them to a "T."
Harris said, however, that the river would suit him to a “T.”