Love the armless sofa-bench! Try using it in the center as a "cross-linking" piece between the sofa and chairs. That way people can sit on both sides of it, conversing with the sofa people and the chair people. Put the chairs where it is and use the side table from the left of the pic, or the one with flowers, between them. You'll need one huge rug to anchor the space or two smaller, and very similar ones, to put on either side of the blue sofa-bench. The other two side tables are too tall and too short for your seating pieces, so ditch them. Use your coffee table between the sofa and armless sofa (18" away from each of them.) Here's a chart for judging table and lamp heights as you'll need to work on that for balance. Please try this arrangement, and post by commenting and using the Attach Images button to add pix of the update. We'll tweak it on out from there.

Upholstery and egg crate type foams will not be rigid, and may not be flame retardant. EPS and XPS construction insulation foam are lighter weight, more rigid, and could be covered with batting and fabric. They come in 4'x8' sheets, but can be cut with a utility knife into smaller sizes. They are available from a big box home improvement store like HD or Lowes. Not sure if they are flame retardant. Depending on how large a piece you are mounting, I would think they may be light enough in weight that they could be mounted to the wall with 3M removeable hangers of some sort.

I dunno, do you like this (from our recently remodeled livingroom w/open entry area? Like Kah says, the easiest place to start is from color -- at least that's what my designer wife did first.

you are thinking about the space right now. it looks odd, no doubt. 1) when you fill in with the furniture, just make sure the three steps are hidden. 2) consider three potted plants of different height, so the rectangle is complete. 3) glass bowl with Siamese fighting fish for each step 4) figurines of different heights 5) 2inch square wooden bar on each step up to the top of the rectangle. and complete the entire rectangle if you want with a smaller rectangle open in the center my suggestion is, there is a reason why this space ended up like this. it will look great once you put something there. things like these are what make your house unique.