Without knowing how that wall relates to the room, it’s a bit hard. Some ideas if suitable for the space.

The only problem I see adding a bar with a sink under that window which is recessed about 2', plus the additional counter depth you be 4' away or near that, you will never get to open that window without climbing on top of the wet bar, even for cleaning purposes. That said, they also have to keep in mind what they want to spend on this because adding a wet bar will require the whole process, i.e, drawings, permits, and cutting up walls/ceilings to run the drain pipes, water lines, GFCI outlets, etc. and that basement looks like it was finished not that long ago. P.S. In the images I posted, image #1 was a new basement design so everything is open... Picture #2 was a modification to an existing basement, the location of the wet bar, there used to be a laundry room which was relocated to the 2nd floor, so the drain and water lines were already there and needed minor modifications for a sink hook-up.

You could hang curtains that would fill the wall space between the windows and on each side of the windows. Curtains can add texture or pattern, and make the acoustics feel better.

IMO TP is never a decor item so remove that little shelf thing and hang some art there . As for a mirror I do not think 2 is the answer.I would do a corner medicine cabinet this one is from Amazon that is it for that spotit has a mirror and wpould look right above the sink. I never undestand the need for a shower when there are no bedrooms nearby so when the time comes to redo this bathroom I would eliminate the shower