Try Dominicks Finer Foods. There is one on Grand AVE about 1 mile west of Great America. It is about $30 versus the $39.95 at the gate. IMO must rides include Whizzer (if it is open), Viper, Raging Bull, Demon, (OH HECK ALL OF THE COASTERS),Deja Vu might not be open yet but if it is DEFINITELY RIDE IT, Giant Drop (if you like freefalls, DONT MISS V2, There are some good flat rides too if you like those. They are strewn throughout the park. Drop me a line I may be interested in meeting up with ya if and when you go. That is of course if you want. If not ENJOY your day at Six Flags Great America. If you want a good eat place in the park (there are plenty just outside the gates like Burger King and McDonalds) I would have to recommend the Crazy Buffalo Saloon in the Southwest Territory. They have a wide selection of foods and even a buffet which is really good considering you are in an amusement park. The pricing though is on the more expensive side for anything other than the buffet. Personally I like the buffet. Plenty of choices and usually good food....unless they over cook the roast beef which tends to happen more often than not.
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