Paint the foundation white to make clean looking and gives a waterproofing at the same time.
Frame the window insides and trim the edges while at it.
Frame the door and make a framed edge to hide the roughness and sharp edges.
Use rough cut 1x6 fence boards to keep the rustic look and let them weather naturally, not paint the wood.
In about 6 months it will age to a silvery sheen.
The roof underhang I would put up a faceboard or fascia will add a whole different look but still be rustic appearing.
Weather appeal add gutters and leaders to move rain/snow melt away from the building.
Some 24"x24" pavers placed in the doorway and a small pad area outside the door area to put some chairs and small table for sitting and catching a afternoon breeze...sweet.
Some hanging baskets of flowers, greens can add homey but not be a ton of work and upkeep...color pop out at you if you use contrasting seasonal growth or even some plastic flowers look nice for a long while.
Some easily made screens in the windows would protect your tack and straw or feed stored from birds nesting and pooping on so much. Also allows air flow so reduces potential of moisture mold ick...
A piece of plexiglass or even some window film used on old single pane windows for added insulation would work to winter weather protect...
Some wall decoration to tie together house, outbuildings and yard decor would add some finishing touches.
I don't know about painting the walls as that then to me is then a continual maintenance thing.
The roof, unless it is rusting I would just leave it alone.