Personally I'd go for Roman blinds hung outside the recess as they won't visually fill the room. I'd go for a stripe, you could stick with blue and white although choosing a fabric with another colour as an accent would be nice and then repeat for cushions.

i might also paint the radiator in the wall colour

It's a lovely room. I'd go for Roman blinds too. Stripes or a simple checked pattern, like in the examples below, could work.

You could use the fabric that matches the wallpaper for a cushion cover. I went for blue, but you could pick another colour from the wallpaper to use as an accent (as Minnie mentioned too). I would change the bedding for plain white.

I like both of Carolina's choices, they appear to be linen which I'd also go for.

i was looking for a close up of the colours of your wallpaper and came across this ready made moodboard! I also like the idea of a ticking stripe and the sand fabric (on their website it's shown as mustard) is quite nice for cushions.

Forgot to say the room looks lovely and very calm :)

Lovely room. Has a nice country vibe. I can’t add much more than the suggestions above, but my first thought was a Roman blind, or curtains, in a plain natural cotton or linen similar to the walls colours? I do think roman blinds look lovely in a simple check too.

I meant the pink floral bedding :-) But only if you're going for perfection, I'd change that to white.

I think you have enough colours currently and the room has a very calming palette - another additional accent colour would detract. Ian Mankin has lots of choices for a Roman Blind fabric that would work for you.