A plant shelf would look gorgeous, add glass, frame the hole add one or two shelves depending on the size of the hole, the shelves , these could be glass or timber, pretty pots and lush plants , could look gorgeous and add a serene look for your room.

You'd be way , way better to put up a couple of pictures , and whether you are wanting to 'hide' the interior view or exterior . Most people would need to know the style and colours of your place -- as dreamer and julie say , you can use leadlights , shelves , plants , art , wood , maybe something like a porthole or a fake wall or a waterfall or a big slab of concrete painted pink may suit ?

I'd just brick and plaster it unless it can be made into a door or window, etc.

I agree with MB design, either it will cost approx the same, a photo of how it is situated in the room would help

Yeh I think brick and plaster it.