You could put some colorful pots with trellises and grow vines on those. Grapes grow like crazy, but they do go dormant in winter. Roses, same thing. Morning Glories, same. Some vines are invasive and will make enemies of your neighbors. Only problem with pots is the plants will need regular water. That rosemary is drought resistant. Many other plants are not. I don't see how you could hide a drip system there, so watering would take a big commitment. You could become a tagger, and just paint faux vines on that wall. Green cushions in the chairs and umbrellas could help.

Just get straight glyphosate - it doesn't have to be RoundUp specifically and there are less expensive brands of this herbicide on the market (Ferti-lome sells a product called Killzall that is less than half the price of the equivalent concentration and size of RoundUp). If you do go with the RoundUp, just select the Weed and Grass Killer - no need for any other additions to the mix. As I noted previously, you may very well need several applications and it is suggested to allow at least 2 weeks after the last treatment before reseeding. And it is too early to reseed - too hot and too dry for most northern, cool season grasses to germinate well. Generally, the ideal time for fall seeding is early fall - September to early October, depending on your specific climate. And any cool season turf grass blend recommended for your area is fine. Most independent garden centers - not the box stores - will have blends that are appropriate for your area.

I think this is going to turn out to be a learning experience for you over the next few years when you see how huge that grass is going to become. In spite of people saying you must move it away from the fence, it is much too close to it and the bed it's in is much too small. One plant should not be closer than 3' to 3 1/2' to the fence. Also, there is no point in placing two like that. Isn't there another corner of the back yard where you could use it to balance out the overall scene? ...Or somewhere else? If you look how big and fat the plant is in my illustration, which I'm not exaggerating, you'll see that the current placement is not going to work out. If you want to add color in front of the grass, increase the bed size large enough to accommodate more plants. You could expand the bed either to right or left along the fence. Or you could make a quarter circle bed that goes from side fence to back fence, and encircle the entire front of the grass with a lower perennial. Hybrid daylilies might work out well as they take full sun, are easy, and can be very long blooming. Or, you could do annuals.

What is your vision/description of a "flat space to play?" What's in it? How is it used? Many houses with this type land configuration have walk-out basements. Is there any possibility you'll be creating future access to the basement from the back yard? What kind of budget do you think you'll have for this project? Retaining walls are expensive and the budget might limit where your thinking goes. A mid-level deck seems more inviting to one who is at the first floor level. The ground is a long way down ... a long way for many of the activities that decks are good for. Plus, it seems that the elevated position offers some dramatic view potential ... not something to automatically suppress. Raised planters at the fence is not practical. What is needed is tall, screening plant material. The yard is small so it's going to be eaten up quickly.