As with all such idea, it is McMurphy who is the person behind the fishing trip idea, and it is he that manages to persuade the doctor that it would be therapeutic to take some of the patients out for a fishing trip and get them out of the insitution and enable them to experience nature and some fresh air. Consider how the novel tells us that he managed to gain permission to go:
He requested another pass after telling the doctor he had some friends at the Siuslaw Bay at Florence who would like to take eight or nine of the patients out deep-sea fishing if it was okay with the staff, and he wrote on the request list out in the hall that this time he would be accompanied by "two sweet old aunts from a little place outside of Oregon City." In the meeting his pass was granted for next weekend.
However, when we look slightly closer, we see that McMurphy has gained a kind of control over the doctor and the doctor himself, as he accompanies them on the trip, is slightly overwhelmed by the presence of the prostitutes that McMurphy organises to accompany them. There might be an ulterior motive therefore to the doctor's ready agreement with McMurphy's suggestion. Overtly however, it is clear that the fishing trip is considered therapeutic because it is a way of getting the patients out of the institution and to give them a chance to breathe fresh air and be out in nature.