Ideally, sitting should be physically comfortable--benches with backrests, well-contoured chairs. It's more important, however, that it be socially comfortable. This means choice: sitting up front, in back, to the side, in the sun, in the shade, in groups, off alone.
Choice should be built into the basic design. Even though benches and chairs can be added, the best course is to maximize the sittability of inherent features. This means making ledges so they are sittable, or making other flat surfaces do double duty as table tops or seats. There are almost always such opportunities. Because the elevation changes somewhat on most building sites, there are bound to be several levels of flat space. It's no more trouble to make them sittable than not to.
It takes real work to create a lousy place. . . .
But it can be done!