The #3 summer fun activity is one anyone can do. Sing loud into a running fan to hear your voice sound really strange. If you have several children they can all take turns trying different songs or sayings. This should give you a good 30 minutes of laughter.
When it looks like my children are loosing interest in this activity I go over and ask a question to them in the fan. They follow by answering in the fan then the fun starts all over with them asking and answering questions to the fan.
To get even a bit more fun time out of our box fan I will say lines from Starwars into the fan. A simple "Luke, I'm your father" is enough to get the children to try saying voices from movies into the fan (giving me a bit more free time to work on other things).