Water Wiggle! Try not to get whipped and maimed by this thing.
In the same vein, and perhaps slightly less malicious, is the Fun Fountain. A more tame alternative to the Water Wiggle, this still could be traumatizing for those with a pathological fear of clowns.
Then again, if you prefer to live dangerously, what's more harrowing in your backyard during the summer months than ye olde Slip 'N' Slide?
Of course, not all summertime toys are water-related. Relive the simple fun of the classic Frisbee!
Here is WHAM-O's Super Balloon. I don't know if I recall anyone in my neighborhood having one of these, and I have a hard time imagining that users didn't get bored with this after five minutes. Feel free to prove me wrong in the comments!
Lastly, an all-time classic toy I've never been able to master for very long--the hula hoop!
Of course, all of these pale in comparison to everyone's childhood favorite--Log!
Now get away from whatever screen you're reading this on and GO OUT AND PLAY! IT'S SUMMER!