Help with living/play room

We have this room off of our kitchen. It’s used as a living room/play room. (We have a more formal living room elsewhere). We’re renting so UNFORTUNATELY cannot change the horrendous wallpaper or gray window treatments. The couch is something cheap we got at the thrift store. We move a lot so don’t want to invest much money in this space, but I’d like to distract from all the gray. I could get a new rug but a large one is expensive and I’d have to store this one somewhere since it came with the house.

I can sew, so I’m thinking of making some throw pillows that are colorful but not too kid-ish. I’m thinking linen since it’s comfy (and the couch isn’t really). Thoughts on these fabrics? Or any other recs? I don’t want anything super bright or saturated. It’s just not my style.