I live in Chicago and have run the marathon once. Other people may have other opinions but I would suggest that O'Hare is too far away. It's at least a 45 minute el-train ride to the city, that is if there aren't any hiccups along the way. Sometimes trains get delayed, etc. I have a couple of friends that airbnb apartments for under $100 (maybe $80-90), so there must be other ones around. I would check into that if you are on a budget. Then you can walk to the el-train that morning.

Don't know what kind of food you like but my favorite place in the city is Avec. I also love Rootstock, Longman and Eagle, Handlebar, Hopleaf, Yuzu Sushi. All places you can get a nice meal and a nice drink with reasonable people. But there are plenty of other good spots, good taco places, pizza places, cheap eats, BBQ, and expensive restaurants..what would you be looking for? Maybe pick where you are staying first then figure out what is close for food. There is good food everywhere. Ooh, this is a really really good list of reasonable places..   http://www.michelintravel.com/selections/2015-chicago-michelin-selections/   I've been to a bunch of these.

Must sees? For me it would be walks around the neighborhoods (how about Lincoln Park, Wicker Park, Andersonville, Logan Square for starters?) but also Lurie Garden, The Art Institute, the lakefront, depends on what you are into. http://chicagoist.com/ is a good website for local stuff.

Hope that helps. You might also post in the main forums to get a better response.

Hey, thanks Joanne for the comments and resources. I'll look into your ideas. Question - does the train from ohare become overly full marathon day. In Boston you have to reserve your seats....

No reservations as far as I know. Found this article.. http://articles.redeyechicago.com/2013-10-11/news/42949317_1_ventra-chicago-card-o-hare-blue-line  It is talking about especially long lines in 2013 due to a change in CTA fare cards but sounds like other years have also been busy. O'Hare is the starting point for the blue line into the city so I can't see any reason that you would have any trouble getting on the train. It will probably just take a little longer to get on and then make longer stops along the way. Just looking now at it says 53 minutes from O'Hare to downtown. So, to be safe I would double that to give yourself two hours from O'Hare to account for any trouble along the way. And maybe then some more to be safe.

http://www.transitchicago.com/default.aspx