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I would try to find out information on some Park District spots around the city. You fill out an application for the space and I think the cost is pretty cheap!

park districks are usually free if your not disturbing anything

You might also, be sure the park has some sort of cover, in case of rain... in the summer less of chance on the Great Plains, but best to be ready for any contingency

Technically the Park Districts are not free. You are suppose to have a rental agreement. Also, as with any government agency, they are sometimes difficult and slow with paperwork.

Check out CD&ME and some banquet halls that offer outdoor space for ceremonies. The great thing about a banquet hall with an outdoor space is that there is an immediate option for bad weather.

Have fun wedding planning!

Chicago Park District is not free. We're having our wedding at the South Shore Cultural Center in September 2011, but it's half, or more, the cost of some other venues in the city. Check the park's Promontory Point as there is an area on the north side that can take chairs and canopy for a ceremony. The dining hall can hold 100 or 150, I forgot, but all the doors/windows open, so you feel like you're outside all the time. It has one of the best views of the city.
Otherwise, if you're in the burbs, check out Seven Bridges in Woodridge. The owner is a family friend and she's been doing banquets for twenty years, sharp. Also, it's on a golf course, so there's plenty of outdoor space.
If you need to go the other way, there's Belvedere Banquets in Elmhurst or Elmwood Park--forgot, but hear it's gorgeous and has outdoor spots. A co-worker, who plans weddings on the side, is getting married there this July.
Oh, another family friend has Royalty West in Willow Springs. Great food, nice setting, and they have a gazebo for ceremonies and a nice pond area for pictures.
Hope this helps! Good luck!