Here’s a great example of rustic log benches in Red Butte Gardens in Utah. Half-sawn logs are surprisingly comfortable but, of course, you need to get the logs. In less urbanized areas, you can generally find a commercial sawmill, or a woodworker with a portable sawmill, to cut the logs for you.
The beefy uprights of these benches are sunk into the ground, but in a more humid climate, you’d want to take additional precautions to prevent premature rotting. Some feel that placing gravel in the post-hole promotes better drainage than filling the hole with concrete, but both work. I prefer the gravel because it’s less labor-intensive and doesn’t leave concrete “stumps” in the ground, should the bench be removed. You may also want to treat the post-end that will go into the ground with a non-toxic preservative.
The easiest way to fasten the seat and back to the uprights is probably with log bolts. Dan has used plastic-covered cable threaded through copper pipe to suspend our half-sawn log arbor seat. The seats of the Utah benches are cantilevered, but one could also construct a frame that included front legs. The advantage of the cantilever is fewer holes to dig and less potential rotting, however this design requires more careful engineering.
These types of benches are perfect for woodland and farmhouse-style gardens, where structures made of indigenous materials blend easily into the scenery.