Summer preparation gets less attention than winter because the failure is discomfort rather than damage. But an air conditioner that dies in the first heat wave is a two-week wait, because everybody else's died the same day.
Before you need it
Run the cooling system for an hour on a mild day in May and listen to it. Change the filter. Clear two feet around the outdoor unit and hose the fins gently. If it struggles, you have found that out with appointments available rather than in the middle of a heat wave.
Water and the outdoors
Turn exterior faucets back on and check for splits from the freeze. Test irrigation zone by zone before trusting a timer. Service the mower — oil, blade, air filter — before the first cut rather than after the third. And check the grill's gas line, which is the one seasonal item where the failure mode is not inconvenience.