I love a blank slate to start with! Looks like you have a big backyard. The first thing to do is to paint the duct and the ac unit white to match the house and paint the back door a vibrant color. You may consider moving that little pond to a back location in your yard as a focal point.

Here are some inspiration pics and a little sketch. House projects are always expensive no matter what you do, so treat it like an additional room to your home. Plan it out and do it in phases. The better it functions and it is the look you want, the more you will use it, even for that morning cup of coffee.

My suggestions:

Raise the deck to meet the top step coming out of your house. It will delineate the deck/patio area. I sketched an arbor/awning over the deck area for shade.

Instead of adding a fence around the ac area, build something interesting. Add a cantilever bench off the wall. To access the ac area, enter from the right side (where the pond is now). The size of that area could be large and gravel around base of ac unit for easy access to everything.

The combination of concrete wall and wood above is a nice warm soultion and very attractive. Match the planter in the foreground to the bottom of the wall - concrete (?).

When you design the steps off the deck, make sure the depth (tread) of the step is deep enough for a cushion or narrow chair. Great for hanging out and eating ice cream!

For the access from the gate to your shed: Concrete blocks with gravel in between and same gravel at base of deck stairs and under ac base.

Don't forget lighting especially at the planning stages.

I like that you have several places to have fun with plantings and play areas. Plant a vertical garden on the fence from the gate. Hang your herb garden right outside your back door on that wall. Set up a bocce ball court, horseshoes or bean bag toss on the right side of the deck. And then invite me over for those ribs!

Simple and soothing. You could eliminate the concrete planter on the deck in the foreground and do a recessed planter like the pic above. It will also make the deck seem larger.

Great idea to break up a solid wood wall, add frosted glass!