My purpose is to see a picture postcard city en route to ORD, so if we can get a bus into the old city to a decent restaurant and maybe a museum or see a little of the city I will be happy.

Alex0683de kindly noted in the StarAlliance forum that DUB-yul-ORD is almost direct, so the feasibility of a stop depends on the flight times and layovers.

Could anyone suggest a minimum and preferred amount of time to experience just a little of the city? I would hope for 2-3hrs in the city, so where do I find out about frequency of Aerobus, rail or taxi services?
Based on the info above, maybe a car service or taxi each is the better option for a couple. If it's a price-fixed service, what is the customary additional tip?

We will bring heavy coats and hats I guess, as we hope to stop in Chicago and will need them anyway.

Perhaps there are areas close to the skybus stops that we could see?
Thanks again for your advice, for all of the questions in this thread.