I'm flying into Chicago on July 30th for family stuff, flying back home from there on August 19th (yep, long trip).
The first thought was to drive to Montreal from Chicago, but it looks like no one else is doing that, and going alone is both not much fun and not very cost effective. Flying is far too expensive, so that leaves the bus and/or train.
I like the train better, but the only way to get there is to take the Lake Shore Limited to Albany, then the Adirondack to Montreal. On the way there the train pulls into Albany at 2.40 PM, and the once-a-day run to Montreal leaves at 11:05 AM.
Solution - Greyhound Bus from Chicago to Montreal, Amtrak on the way back. It'll be about $100 one way on bus, and about $120 for the return on train. I've got the time, so why not.