I like to have my garden rototilled at least once a year and often again in the fall so permanent structures do not work for me in the garden where I grow cutting flowers and vegetables. I use raised beds that I make with my rake in that garden and plant everything on the level ground after about mid May (Northwest Washington state) when it becomes warm enough and dry enough for my plants.
A permanent raised bed is high maintenance but worth the work if your health is good and your area is not plagued by crab or crack grass which will crawl right over your bed structure in the off season to invade your garden.
Because wood is subject to decay it is often treated with minerals that can kill or severely retard your plants within the bed. Be very careful about that. I am cheep and tend to use scavenged materials so this becomes a real issue for me. Right now I am lucky to have a large pile of raw cedar for my hard structures and posts. My husband used to work in forestry and had permission to scavenge landings for posts. We have used and reused that pile of wood for everything from a chicken run to pea fencing to raised beds. It is a good size pile of wood long enough to sink about two feet into the ground and still put at least 6' of fencing on. They are big enough in girth that roosters and cats love to sit on them. I am not sure we could get something like that for free any more. Nothing has worked for us like cedar. Moss has not been a problem and rot is minimal. It splits easily (easy if you have the right tools and a teenage son) and still has a fresh cedar scent and bright color look when cut open (the posts are easily 20 years old). Pine tends to get slimy in the rainforests of the PNW so it is often treated with inorganic, garden unfriendly finishes and still does not last as long in the garden. However while it is fresh is can look better (i.e. more finished) then the raw cedar. The one major downfall of cedar is the splinters and slivers it tends to jam into your flesh. I have a raised blueberry garden that is edged with cedar posts. While the posts look inviting to sit on while weeding, it is a bad idea.
I also have a large collection of some kind of white stone embedded with fossils of tiny sea shells that at one time was at the bottom of the original porch where we live. They have been cut into slabs but still have the look of (rock?). In the PNW they turn greenish if they are not walked upon regularly.
I have an amazing amount of cinder blocks stacked behind the pump house and have often thought of using them to build a raised garden for herbs or strawberries. It is just that they are so ugly. I am sure they would work fine. I keep thinking that they could be laid on their sides and things could be planted in the holes. However, as long as people keep knocking down old brick structures and letting the Prince and I cart off the old bricks, I will keep using the bricks in my garden. They are so much prettier in paths and edges. I do use the cinder blocks to make glass covers for different small areas that I do not want to drown in the rainy seasons. We have a collection of old window frames that we lay on an angle across cinder blocks that have been set on end on one side of a bed and on edge on the other so that the glass sets at an angle to the east across the bed. When it is not so wet the glass and blocks are removed from the garden. I have also used the cinder blocks to make stair-step type areas for potted plants outside. That is probably the least ugly use of cinder blocks.
What ever you choose for your hard structure you will want to be able to tend your garden without stepping in it. You should be able to stand, sit or kneel comfortably on the hard structure (edge) of the garden. The beds can be just as long as you want but I would not go any wider then three and a half feet (I have a rather short reach) so that you can tend to the plants in the bed without ever putting any weight on the soil. You should be able to reach about two-thirds of the way across the bed without putting your other hand down (the weight issue) to rest against while working the bed.
If money and materials were no object then I would plan my structure with places to fasten PVC hoops for plastic or a net cover to extend the season on both ends. The hoops or triangles can be covered with garden grade clear plastic in spring to protect very early plantings from most frost and weeks on end of heavy rains. Covering the structure with mesh in the heat of summer will keep the lettuce happy and the moths off of your broccoli. If your tomatoes are still green in September the plastic hoop-house just might bring them along. You can also have salad from the garden for thanksgiving and maybe Christmas if it is grown under the plastic hoop-house.