Go swimming, play in a sprinkler, water guns?, play sports in your back yard (volleyball, badminton etc...), sun bathe while reading magazines and enjoying an ice cold beverage, slip-n-slide, water balloon fight...so many things to do.
In our part of Australia, summer usually means temperatures of 30 - 45 degrees. With those temperatures, all one can do is stay indoors in front of a fan/airconditioner.
You should totally hit the beach. There's nothing more refreshing than a good, cool swim on a hot, sticky day. After you're done swimming, then go with the AC idea. xD
Swim at an neigborhood pool, or backyard pool Play a video game Play Truth or Dare Go to an arcade
Stay indoors w/ the AC on & have some lemonade! :P