Lake Geneva is a tough call.... used to drive that way twice a month in the 70's and 80's...
It's about 90 minutes from O'Hare Airport on any of the various ways involving US12 (I-94 to IL173 to US12, I-90 to IL 53 to US 12, I-90 to IL31 to US 12) and you'll have lots of stop lights & traffic.
My first choice would probably be I-94 to WI-50, but I have no clue as to how built up it is today.
Heading across I-80 to I-39 and I-43 may actually save time and gas with the local speed limits and all the stops & starts, and certainly will be faster running and cheaper fuel... But it is out of the way, and if you hit Chicago at the right time, the tollways do run fairly well now that there is open tolling. EZPass worked the last time I checked, so you might want to get one before leaving if you don't already have one.
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