I used some old 2x4's in my garage and planed them down with my electric hand planer. I had no idea how pretty a 2x4 was inside. (A lesson in outward appearances I guess.) With a couple thick coats of marine quality varnish, this swing will hold up against the elements for years!
The swing was designed to hang from four 3/4" nylon ropes inserted through the holes in the top and tied off under the swing. I reinforced the swing with two strips of 1" poplar, glued to the three planks and fastened with a nut & bolt on the middle one. It makes for a very sturdy and safe swing.
I routered all the edges of the swing so there would be no sharp edges and to eliminate the potential for slivers. I imagine slivers from a swing being quite unpleasant! I routered out the top and bottom of the rope holes to protect against rope wear from excess rubbing after years of use. This was a super fun project to work on, and again look at how beautiful the wood really is! It was a fairly simple project, but it was solid and turned out great.