There are a number of ways to cool down a hot backyard, including installing a shade structure like a pergola, parasol or shade sail.
If budget and space allows, a swimming pool or plunge pool are both instant ways to cool down a hot backyard. But a water feature or water fountain will also work to cool down the space if you're on a smaller budget.
Choosing heat-reflecting colors, like cream or white, for your outdoor furniture will bring a fresh feel to the space, while using cool-toned plants in your garden border or deck planting ideas will also create a an airier feel.