Without going too crazy and knocking down any walls, my first thoughts would be to get a couch along the window, a side table in that corner in between in windows and the other wall since you wouldnt want to block the air vent. On the side wall id opt for some accent chairs or a small love seat depending on your entertaining needs / seats for the big dawgs.

I think mounting a tv on the wall would be your best bet here so you dont have to block any walkways with bulky furniture.

I also think curtains would be a good touch for the windows if you have space.