Navy Pier is definitely fun in the summer. Fireworks on Saturdays.
We also like going to the Signature Room restaurant at the top of the Hancock for a drink - great views.
Take a river boat tour.
Millenium Park is great.
To eat: Ed Debevics for lunch...Lawrys Steak House if you like Prime Rib, Ginos East or Uno's if you like Pizza
There is so much more, if I remember, I'll post!
ps - Cubs games are awesome, and if you can't get tickets, just going to the bars and restauants across from the park can be fun too.