Reduce fertilizer applications to allow plants to harden off before winter. Continue to water deeply – less often.
Continue to dead head spent blooms on perennials and annuals to encourage more flowers.

Harvest vegetables frequently. The more you pick, the more you get!

Watch for insects like Japanese Beetles (pretty green bugs). Pull them off plants and dispose.

Perennials can be divided now. Double your plants for next year or give them away! 
Signs that division is required: center of plant has died; plant is no longer flowering as profusely; plant is getting too large for the space.

Move houseplants into the shade if necessary to accommodate them to inevitable lower light conditions.
Collect and dry seeds from plants now for next year’s propagation.

Consider not watering the lawn for a 3-week stretch to induce dormancy. Yes, it turns brown and looks dry and dead, but underneath, the roots are growing deeper looking for sources of water. It is natural for turfgrass to have a dormant period during which the plant increases its drought resistance.

Conversely, most evergreens do not experience dormancy and require additional watering during hot dry spells. One inch of water per week is recommended, and don’t let the soil dry out: you want to build up a soil moisture surplus heading into winter.  

In the vegetable garden, go for another round of cool-weather leafy vegetables like lettuce, spinach and bok choi. By the time they germinate, the weather will/might be cooler, and they’ll be big enough to not get smothered by tree leaves that may begin to drop.