A permanent bench can also be made of one treated (termite-resistant) 4X4, and a 2X8, 2X10 or 2X12 plank, or combination of boards to make an eight to twelve inch- wide seat. Start with a treated, cedar, or redwood post at least 4 inches thick. Cut the uprights to about 36-48 inches long. Cut a notch in top for a 2X4 or 2X6 cross member. Although it doesn't have to be perfect, it is helpful if the cross member nests in the notch so the top is flush and square. Junk wood, or scrap wood can be turned into some nice looking garden objects. If you have kids, you do the cutting, and let them help with the rest. They can nail everything but the top. (Bent nails on the the top can tear clothes.)
I do not like to cement upright posts into the ground. Posts rot easier when they're cemented. I usually will drive a few nails into the bottom sides of posts to dramatically increase the surface area. Upright posts need to be no more than about six feet apart for an eight foot long bench. I suppose two 4X4's could support a span up to ten feet if you use larger lumber, but the bench looks best when it is between four feet and eight feet in length with no more than a foot or so of overhang. Once posts are tamped into the ground with side nails in place good luck moving them around! The small cross member needs to be nailed before you set the posts! (Notice that this little piece of wood is even with both the top and one side of the upright post.) The long cross member needs to be tacked lightly, the upright posts and the long cross member leveled, BEFORE everything is tamped hard and nailed. Once you get it leveled, tamp the posts in very solid(great kid job), and finish nailing the boards on.