You know what, I would move in, place your current furniture and live with it for a bit, then you will have a better idea. We jumped the gun on a couple of rooms in our current home which we moved into 4 years ago, and I wish I'd waited a little bit to get how we use the room, etc. That triangular area doesn't have to have anything on it...it could stand alone as just an architectural detail. We have some of those in our living room, & I've put nothing on them & it looks fine....depends on if you feel the need to decorate up there. I agree with Shannon that I'd put an additional chair, if you use one, on the side opposite the window to balance that area or if that piece housing their tv isn't staying, you could put your longer sofa along that wall, thus opening up the beautiful bay like window & put the chair on that side for reading. Where are you putting your tv...is the large piece staying? You could put your tv above the FP, maybe. If you have wood blinds, you don't really need drapes, unless you want the softness of fabric. I would leave that decision for a bit. Christopher Lowell (from hgtv, etc.) has a book on the seven layers of design & he advises window treatments, accessories, etc. as probably the last step. It is a beautiful room, BTW, and I hope you'll post as you decorate it!! Good luck!

It looks like the best thing is to start with a new 3 seat sofa. Since you have the TV above the mantle, we assume you want your seating positioned that way. Then you will want two chairs to go on the wall directly across from the slider. You also may like a rug? Below you can get a lot of great ideas from designs of ours. Note the furniture, rugs, end tables, pillows, lamps, artwork, window treatments, coffee tables, and various other decor accessories.

Not a Pro. You said this was a lake house. Do you use the fireplace for heat? I would move the fireplace and place a very large piece of art there. I would also hang the tv and hide all the wires and add a shelf for all the boxes and remotes. Nice and neat.

The two stories provide airiness, and you don’t decorate the air. So unless you have some massive piece that needs that height to fit, decorate like you would any other room that was a typical ceiling height…keep it at people level and just appreciate the vastness above.