You can replace wall hangings with Holiday themed items, tack garland on the walls (perhaps in ornament or tree shapes), change out rugs and throws to holiday colors and put holiday colored lights in some light sockets.
Can you take away a piece or two from the sectional to help it fit ?
I"m Linda and I suggest that you focus on the tables. Use bright colors and find accent pieces that will draw attention away from your furniture and to the items on the tables. I usually put away my normal decorative pieces at Christmas then replace them with holiday items. By doing that, you're not adding items, you're just temporarily replacing them. I often put the normal items in the boxes that hold my decorations so that they are easy to find when the holidays are over. Wishing you the best and happy holidays!
Use them as the focal point/anchor. Place other pieces around the large ones and accessorize sparingly. Make the large pieces be the reason you pare the number of items which will actually make the room look visually larger.