I've done most of the above advice (got a Japanese Maple in the hidden corner thanks North Leeds), am training grapevines up the back wall, lots of biggish similar mixed planters, I have some tomato plants climbing up the left wall and they are providing interest. Plus lots of herbs as they add both nice smells and are useful for cooking. I just need to figure out something to do with the floor which is old, horrible, cracked concrete. I'm considering painting it in a patterned effect or just tiling it, Mediterranean style. Bigger planters look much better than the small ones I originally had, it adds more impact and obviously allows for bigger, leafier plants.

A larger picture frame. A single tall candlestick in a metal to match the mirror frame ... +++. Brass rather than pewter though would brighten up the area more. Alternatively a lamp would brighten up the corner but to make it fit move the sideboard so that the mirror is off centre. If you were feeling extravagant you could get a custom piece of stone made for the top. (Gloss black, polished stone and brass are a really classic combination.) Route through your favourite glassware, pottery, porcelain etc. and find a few complimentary pieces and make a balanced arrangement with them.

I think you'll be pushed to fit anything else in If you're adding a sideboard. Do you have a dressing table in the walk in wardrobe?

Some leftover roof tiles were used to build this wood store on the side of a garage (wood is from dead trees felled on the same site), which is practical and adds to the look of the garden. If you were to widen the front by removing some stone would it be large enough for similar, with a sleeper across the front, or even a bike-shed/park?