I've been there a bunch of times. The Superman ride isn't really that great, Raging Bull and Vertical Velocity are much better. The gifts shops seemed pretty well stocked with the usual stuff, didn't check the prices. There's a bunch of restaurants/stands throughout the park, you can probably get something good, but does it really matter? You're gonna be on roller coasters all day, it might come back up. They opened a new Water Park edition next door (admission is included with the ticket to the actual park) last year that's supposed to be cool. The park is packed on weekends, when I was in school I'd usually go mid-week, much less crowded. If you pay a little extra you can get something that allows you to move to the front of the line on a certain number of rides, totally well worth the money as some rides can have a 2-3 hour wait, you use those and you can hit everything and maybe go to the water park too.
Also the park isn't in Chicago, it's in Gurnee about an hour and a half outside the city.