I would visit the Art Institute. They have extraordinary exemplars of many genres. And I would pop over to Millennium Park.

Thank you. Any particular reason for recommending Millennium Park?

They have some fun and unusual structures.

Have you been to the Field Museum to see "Sue"? The Shedd Aquarium is also worth a visit. Check the Architecture by boat on the Chicago River which is very close to Union Station.
There is even a free self guided tour of the Federal Reserve Bank on Jackson across from the Chicago Board of Trade.
Walk the Magnificent Mile clear up to the Drake Hotel. Take your pick on whether to go up the elevator in the John Hancock on Michigan Ave. or the Willis (Sears) tower 2 block east of Union Station on Jackson.
I'm glad to see that the old wooden benches have returned to the Great Hall in Union Station. They were out for an event when we passed through on 5/17. We got to sit on them when we passed through on 5/29. There are 2 areas of luggage storage lockers. The largest is next to the baggage claim and the other is at the south waiting room which can be a zoo about half an hour before train departure time.

Dukey, although I am far from kid age I love the Museum of Science and Industry. However I would recommend either the Art Institute or Shedd Aquarium. They are both downtown where as the Museum is a pretty far cab ride.

Thanks for everyone's response and thank you Aduchamp1 for explaining further your rec re the park. I must add here that it is much more helpful, to me at least, than the "it's the logical choice" response periodically seen.

I've kinda ruled out Science and Industry at this point. I feel I've kinda done the coal mine, the U-505, etc., although there is something wonderful about monstrous locomotives and oil well machinery.

I've never done the Art Institute and a friend recommended seeing the Thorne Rooms in particular; the Shedd Aquarium I hadn't even considered until now.

As to Union Station, I'll make a point of looking around closely. I'll be in a private RR car so luggage storage, etc., is not an issue this time around.

Thanks again for the recommendations.

To expand on Aduchamp's post, it's not just the structures in Millennium Park, but also the spaces, events and people-watching. Walking through the Lurie Garden is like being in a rural pasture even though the cars are whizzing along Lake Shore Drive and Michigan Avenue just a short distance away. The Crown Fountain is always filled with people wading(?), watching the faces change and running under the spouts.

There's a temporary sculpture exhibit through October and the Grant Park Music is holding its 78th season of free concerts. You can find a map, pictures and schedule of events at the park at www.millenniumpark.org.

Who knew an old railroad yard and parking lot could become such a great place.

Who knew an old railroad yard and parking lot could become such a great place.

Good for you. Around these parts an old railroad yard has BECOME a shopping center with a parking lot.

You definitely have to go to the Art Institute -- if you only have time to see one major museum, it should be this one. Should your visit be on the first full weekend of the month and you have a Bank of America credit or debit card, admission is free. http://museums.bankofamerica.com/ Here is a link to some photos I took there: http://www.fodors.com/community/fodorite-lounge/art-institute-of-chicago-photos.cfm

The nearest 'L' station to Union Station is the Clinton-Blue. Walk one block west from the station and and couple/three blocks south on Clinton (underground entrance underneath Eisenhower Expressway overpass.) To save a little money, you should go to the CVS near Union Station and get a 24-hour pass for CTA -- about $5.50 I think for unlimited CTA transport for 24 hours commencing from the first time you use it.

Walk out on Navy Pier for some nice vistas/city views. The stained glass museum inside the corridors is pretty nice. From that area you can head north and walk part of the shoreline trail passing some city beaches.

Pop into the Chicago Cultural Center (free) and go the the upper floors to see some exquisite interior design and the two stained glass domes (one being the largest Tiffany installation in existence.)

Across the street from there (@ 72 E. Randolph) is Hot Tix where you can get discounted theater tickets for same-day performances. http://www.hottix.org/ If you don't want to see a play, there is no shortage or other nightlife around Chicago -- clubs with jazz/blues and other music venues with generally great rosters. A few of my favorites are:
http://www.schubas.com/
http://www.lincolnhallchicago.com/
http://venue.congresschicago.com/
http://www.emptybottle.com/
http://auditoriumtheatre.org/wb/

Also, there's bound to be some kind of festival on while you're there. Pick up one of the free entertainment rags from a store entrance or news rack. It's a great city with so much to see. Just walk around, explore things that strike your fancy and cram in as much as possible during you 24 hour visit.

Wow, you are truly fortunate to be able to ride a private RR car into Chicago. Which trains will your car be hooked to coming in and leaving Chicago? Are you headed for home or just starting out? I did enjoy my recent Amtrak trip even though it is difficult to sleep in coach.
On the trip east we had an evening meal at Pirogi Heaven which is under the "El" on north Wells. Adams and a couple of other streets are ripped up with construction east of Union Station.
You can also get a city bus near the corner of Canal and Jackson after you get the 24 hour CTA pass.

If you haven't already done the Chicago Architecture Foundation River Cruise, it would provide an interesting counterpoint to the city perspective you are going to get from the train.

Thanks for the additional comments and recommendations.

re the rail car: it is coming in on the Cap Limited; remaining overnight at the station; out the next day on the Southwest Chief bound for Los Angeles.

The big "problem" with Chicago: a lot of things to see and do in addition to all that great architecture.

The Art Institute is "world class". I'm not an "art museum person" (when we go to Paris, we do what Art Buchwald called the '3-minute Louvre'), and I still wouldn't miss it. It's enough just to see

http://www.artchive.com/artchive/s/seurat/jatte.jpg.html

and

http://www.artic.edu/aic/collections/artwork/111318?search_id=2

Thank you, sf7307

BTW, I love Millenium Park -- again, if only to see "the bean" and the Frank Gehry-designed Jay Pritzker music pavilion.

The Art Institute does not have a deep collection but what they do display is impressive.

I always say, just show me the highlights, not looking for a PhD!

Yes, Chicago Art Institute is well worth a day, especially if you do appreciate art. I used to go through a various times selecting a particular gallery. Actually give thought to what style of period of art you most favor if time is limited. Also try and join a tour.

Could be some concert can be found at Millenium Park pavilion as we once found.

Bill in Boston

Another vote in favor of the Art Institute, which I found to be a world-class art museum. You could spend the day there, given its size.

Will also second the Field Museum and Shedd Aquarium, both close by each other. Though a bit away from the Art Institute, they're still closer then the Museum of Science and Industry. Plus, if you've seen the sub and coal mine already, you've seen two of that museum's major highlights (not that there aren't more good things to experience there, of course).

You can access the Art Institute by a long ramp from Millenium Park.Have lunch at the Museum's Terzo Piano, very ggod food and a nice light filled space in the Renzo Piano designed new wing.

Today I was googling the artist Marsden Hartley. That made me want to go to the Art Institute. It's already been very high on my list of art musuems. It sounds like an amazing musuem.

Again, thanks for the comments and recommendations which are much appreciated.

First, a big "yes" to the Art Institute. Now, there's a lot to see there; so if you have any interest in it, they have audio tours of the highlights of the museum, as well as guided tours (look at their calendar online) for those with limited time.

Although you can indeed spend an entire day there, in your case, I feel it might be overkill. Especially in the summer, if the weather is pleasant.

My suggestions:

At the base of the Willis Tower (across the Chicago River from Union Station), you can pick up Shoreline's water taxi to the DuSable Bridge (fka Michigan Avenue Bridge). I'm suggesting this not only for the view but for the time savings over CTA bus.

If you have the time and inclination, I'd recommend taking the Chicago Architecture Foundation's River Cruise - where you'll learn about all those buildings you've just passed on the north and south branches of the Chicago River (plus more). Their boarding area is on the River Walk at the southeast corner of the DuSable Bridge. You'll see the blue awning.

Speaking of the River Walk, it's really nice to just stroll along, stopping from time to time to view the River traffic.

Millennium Park is three blocks to the south of the Chicago River on Michigan Avenue. Right across the street is the Chicago Cultural Center, where you can take one of the FREE InstaGreeter tours of the Loop. You don't have to reserve, only show up.

As I mentioned, Millennium Park is right across Michigan Avenue. The Frank Gehry-designed Pritzker Pavilion is where the Grant Park Music Festival performs, as well as the Downtown Sounds festival. If you take a look at the former's calendar, you will see that they have open rehearsals during the day, when their performances are in the evening.

If you were interested, the Chicago Architecture Foundation's ArchiCenter is across the street from the south end of the Art Institute in the Santa Fe Building. You might wish to view the Chicago Model City.

Dang it. I wanted to post some links and mention one other thing:
www.shorelinesightseeing.com/watertaxis.html
www.architecture.org
chicagogreeter.com
www.chicagomodelcity.org
www.grantparkmusicfestival.com

The Bridgehouse and Chicago River Museum is at the DuSable Bridge. Even if you don't care to stop in (and I hope you do), look at their website, as well as that of the docent mentioned in the Bridge Lift section. bridgehousemuseum.org/home

My purpose for mentioning these other options is not to distract from your original purpose of visiting the Art Institute (of which I highly recommend). And I wanted to give you a number of them.

I just noticed an error in my first post. With the Shoreline water taxi, you are on the MAIN and south branches of the Chicago River. My apologies

We lived in Chicago for 20 years and if I had only one day, it would be the Art Institute. IIRC, summer should be a terrific time to have lunch outside, they do a very nice job and it's lovely.

If you go, please think of me. It's one of my fondest memories of when my children were young and we spent Saturday mornings there at their free children's art classes. We often splurged in the summer for lunch outside. Their collection of Impressionist art, if you are into it, is lovely and lots of other interesting stuff to see. Have fun!