Day Trips - Plainfield,IL

There is a really great website called Chicago Free that you should check out. It shows days of free or reduced admission for many Chicago events and attractions. http://chicagofree.info

You should also consider taking the Metra in, as that is an experience in itself, and girls that age should be able to stand walking from Union Station to many of the places in the area.

God bless you for taking a group of middle school girls out! One of my friends' moms used to do it with us, and now that I see what middle school girls are like, I realize she must have been a living saint! :)

Phillips park in aurora has a water park, a small dinosaur museum, a train ride that is free that goes around the park and a couple of covered places in the park for lunch. you can bring a picnic lunch, eat and swim its a really fun day

http://www.foxvalleyparkdistrict.org/?q=node/35

another fun place is FunWay in batavia they have bowling, skating, arcades, bumper cars etc
http://www.funway.com/

also wasn't sure if you knew but all of the museums etc in chicago have free days each month. the field museum, the planetarium, shedd aqaurium, museum of modern art the art institute etc. and don't forget about brookfied zoo you can do the train there

Picnic on the lake. Day hikes identifying plants. Biking to the library. Bike touring a town near you. Go online and find the historical houses in your area and make a few tours of these old homes.

Yesterday's Chicago Tribune had a chart with all the museums in the area and showed their prices for admission and parking. Some of them offer free days (I recall one or two had free admission for the rest of this month). If you can't get a hold of the paper I'm sure you can find it online - unfortunately I can't remember what section of the paper it was.

I would suggest going to The Grove in Glenview. It is on Milwaukee Ave. My older daughter (21) has taken my youngest daughter (10) there a few times. It is free. They have a building with animals, lagoon with boardwalk, trails, a house that they give tours in. They really like to go! Also, down the street is the River Trails Nature Center. They have animals in cages outside and activities. They have a bunch of trails. They visit these each summer.
Good Luck