There are tons of places in St. Louis and Chicago. Economy-wise I think St. Louis is cheaper compared to costs for taxi's, bars and hotels. But there is 10x more to do in Chicago
Here's just a few ideas ...
St. Louis-wise there are tons of hotels downtown that host formal events and Laclede's Landing - an area of bars and restaurants by the river and Arch is nearby. There's is also Union Station - that has an awesome hotel and is also near the Landing and my fave bar of all time, Have a Nice Day Cafe (or as we call it Happy Day Cafe - its a chain bar and can get old but its where my friends and I used to always go since we were 18.) Or there is also Westport Plaza - another area of bars and hotels.
Chicago is filled with hotels and bars. There are boats and venues off Navy Pier. The W Hotel Lakeshore is a blast if you can book something there - but I am sure they are packed. Cheaper options may be something by O'Hare but then you are limited as far as getting to bars afterward.
If you are willing to go a little more rural, there is Lake Geneva in Wisconsin and Starved Rock State Park south of Chicago or Rend Lake in Southern Illinois - using a state park lodge facility for formal and staying in furnished cabins afterward or a Ski formal, formal dinner and dance and then skiing the next day or the day of is another idea. There are a few ski resorts in Illinois(northern) and Hidden Valley outside St. Louis. Hidden valley is fun if you are new at skiing but its not a Vail or Aspen resort.
Hope this helps a little ... Where are you exactly at?
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