What section of town will you be in? The Mag Mile I'm guessing?
I would suggest the architecture tour on the river boats for starters. Hancock or the Sears (Willis) tower. To be honest, other than for shopping or touristy things, downtown is only so-so. You need to head north into the neighborhoods for a little more action. I'd also check the website Metromix and see what events are scheduled that weekend. Avoid Navy Pier!
When the weather's nice, there's a neighborhood street festival every single weekend here - it's ridiculous.
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