Not every surface needs something. My decorating philosophy is to find a place in my home for the artwork that I love.

With all your beautiful windows, I don't think those walls need anything (unless you find something fabulous you want to display).

I would definitely be tempted to add art. Take your time finding something that speaks to you instead of something off the shelf. If you choose well you can enjoy it for years to come. Also nothing wrong with having the toddler play area there. If it keeps the child within your sight and entertained then that's a good use of space. :-)

I see a toddler. I would place a large piece of toddler art on that wall. Painted on MDF.

I'd hang a small piece of artwork that you love. You already have enough visual information surrounding this area from the other rooms. I would not add a bench or create another purpose for the area. It is really nice to have blank space.

I don't think you need anything there! The way it is, the eyes move across the rooms, rather than stopping at that wall. If you want to hang something, I'd go textural - either a modern take on macrame, or something like this: