Definitely plants!

I suggest a large round or oval rug to counteract all the hard angles. Also soft rounded cushy chair/ottoman.

There are large removable decals you can buy and easy to remove later. Perhaps a forest or lake scene. Whatever suits your personality. The angled wall could be a great feature with a decal. There are lots of fun ones https://www.amazon.com/NISHA-Pictures-Removable-Backsplash-610/dp/B08TB1KHMF/ref=mp_s_a_1_31?keywords=wall+decals+removable&qid=1674222490&sr=8-31

I’d also suggest some nice cozy lighting. There are cheap LED light bars you can stick up on that beam to give a glow. Or use a few lamps.

Please post after pics!