Go to the Beach

Ok so this one is a bit of a no-brainer, but just in case. Everyone should go to the beach at least once during the summer. It's pretty easy to find and get to, I mean c'mon. I personally prefer Oak Street Beach, because it is pretty clean and not too crowded, also it gets some decent waves. Go forth, explore the sandy dunes.

Movies in the Parks

The Chicago Park District does this great thing where, during the summer and early fall, they show movies outdoors. They show different movies at a multitude of parks, some of which can be really close to you. Best of all, it's FREE. The full list of scheduled movies is on their website. So grab some friends, sleeping bags, blankets, and sit back and enjoy the show.

Flash Mob!

One advantage of living in a large city is that there are a lot of public places. What better thing is there to do in a public place than a flash mob. It doesn't necessarily have to be an elaborate choreographed dance, it can be as simple as a pillow fight. I'm providing a link to a how to guide for flash mobs.
Go Forth!