The scenery doesn't hurt, but I think for me its mainly just the overall relaxed mood of the show. The feel of the trio just hanging out and enjoying their days together comes across from beginning to end with this program. The style of humor is non-confrontational and is mostly not about making fun of anyone, but rather just enjoying the characters being themselves. Most shows, even comedies, operate on setting up an issue that needs to be resolved, which inevitably creates tension. The way Roy Clarke wrote Summerwine mostly avoids that approach, so even though many episodes have some central story or issue, the show doesn't build it up to the point where in other shows you would get the big last payoff and then be out. Instead it resolves the story or issue and then sort of slides off.
I watch a couple of episodes of Summerwine just about every night, before going to bed. When I do crawl into bed my mind isn't racing about or focused on daily life issues, its just in a relaxed mood. Perfect.