I echo all the art-on-the-walls suggestions. even blank canvases can be something if you think you might later paint on them. but there's plenty of secondhand shit for the walls all over. you can also hang fabric on walls to create warmth and patterns. I like hanging fabric on doorways and in hallways too, to pass through, because I am a weirdo and like to articulate the visual field to define spaces. people usually furrow their brow at it unless they are also weird like me and then they sort of laugh when they pass through.

also, depending on the floor situation, rugs are cool. usually one at the entry door(s), one in the bathroom, one for the kitchen, and something for the living area.