To start with, I’d replace the ceilings with decent wooden boards. And since your little house has this, you know, cozy homey vibe, I’d add a small fireplace, put all sorts of stuff on the walls like stuffed fish, antlers, paintings, and some kind of a little couch. I'd set up a kitchenette in a pretty primitive, rustic style, and make a bedroom on the second floor. And basically, just fill it up with all kinds of atmospheric junk so it looks cozy and lived-in.