How do I fill space around window?

Planters in between?

Use reclaimed lumber or exterior plywood that can be painted to protect from the elements. There will need to be frames made for the windows to set into. Do some research first before doing this project. It should be a fun room when finished.

If living in cold area and talking about filling in between the posts, I would invest in some styrofoam sheets to insulate then 1.)maybe panel with plywood or paneling. 2.) Or (a bit wacky maybe??) Cover plywood/paneling with leather/naugahyde and attach over styrofoam. Easy clean! 3.) Paint wood with picket fence look. 4.) Or attach long planters/feed or hayrack style in between posts and grow lettuces, flowers, spinach, radishes depending on depth of planters/hayracks. 5.)shiplap also seems to be in style. 6.) Use thin plywood and find tile you like and tile and grout plywood. (Also easy clean).

If insulating and the space is small you may need to use strips of fiberglass insulation to plug up the space. Measure the distance between the outside edge of one window to the next. Go to your lumber store to see if you can find wood strips in that width to "cap" it. the wood may be stained to painted prior to nailing it on the outside or inside to match the remaining area.