You have done a beautiful job of tucking plants into the crevices of your retaining walls. Your house trim color is perfect -- just slightly lighter in hue than the retaining wall blocks. First I would paint the big, flat gray wall slightly lighter than the blocks and slightly darker than the house trim. The concrete walk and stairs I would stain a mottled blend of the blocks and the house trim colors. Next would be to paint the railing black or the house body color. Now, think of the following as creating a light, airy trellise structure from the walk-level face of the (currently gray) wall to about 7' above the top of the wall (using 6x6" posts attached to the wall) with a corresponding line of posts sunk into the terrace level 2' in from the outside edge of the wall-attached posts. The inside posts would be about 6-8" higher than the outside posts. Put 4x4" wood to attach the outside line of posts to each other and do the same with the inside line of posts. Run wood pieces at the top of the two lines of posts from the inside line to the outside line at about a 45 degree angle. They would be cantilevered up at the terrace level. Across the top of the angled pieces create your top trellise boards which run parallel to the two lines of posts. OK. There you have your basic structure. You might want to extend the structure around to follow the street side of the terrace. You could then follow the suggestion of reom10 above about creating a bench-planter on the terrace side. Your spacing of the boards parallel to the gray wall and attached to the outside posts would determine how private you wanted your terrace to be. You might want to soften the terrace "floor" by using something irregular like flagstone with groundcover plantings between the stones. From the latticework on top of your structure you could hang seasonal flower baskets. Plant vines to go up the trellis and spill down the wall. How about planting a row of tall, narrow shrubs along the neighbor's fence above your terrace to help add to your privacy and make your yard feel less 'structured'? Or, how about patio table/s with umbrellas to block your neighbor's view? Anyway, you have a fun project ahead of you.

Your picture display looks lovely with your sofa and will look better if you bring it down because it's a bit too high above the sofa ( unless you plan to keep adding to the picture collection below from where your pictures are hung now) and in that case just keep in mind that multiple frames look best when arranged in a diamond, rectangular or square shape and spaced relatively tight, like 1-2 inches apart. A looser grouping can work for a more casual display.

on the landscape plan - The road frontage is the left hand curved side & the black rectangle is the position of the outdoor fire.

Hey! Great space! The table is too small for the space, it would immediately help if you had head chairs on either end of the table. It would read larger. If you're interested in a new dining set, try a round dining table, seating at least 6 for size. You need something to define each space, a new furniture arrangement could help. I would try rotating the sectional so that its off of the window. Good Luck!