Opinions for a Few Hours in Chicago?

I actually vote for just walking around the downtown/"Magnificent Mile" area. There may be some specific art pieces in the gallery which you can't see elsewhere, and the Aquarium may be great, but when it comes down to it, you can find museums and aquariums anywhere. The beautiful old historic buildings and the feel of a city like Chicago, that you can't get anywhere else. So I suggest you head for N. Michigan Avenue and enjoy the view! It will be a bit chilly (wear layers and a couple pairs of socks), however if you take a break in a coffee shop, that can be part of the experience. Enjoy!

I agree with CrazyLIfe below...
For a few hours, Art Institute and walk around the area.
If you want the real touristy stuff...start with the Art Inst., then head outside to Millennium Park. Walk up either Mich Ave or State St (shopping!), and finish with a drink at the 96th in the Hancock building.

The Shedd is a little further down on the museum campus, is often packed with a line out the door, and is expensive.

I was born and raised in Chicago. If I had a day with no kids, I would go to the Art Institute then walk around downtown. I love architecture and art so they really go well together. You can explore Millennium Park which is right by the AI and then head north to visit the "Magnificent Mile" and take in all the great architecture we have here.

Be sure and get some of that magnificent Chicago pizza!

The Art Institute and the Shedd are both sooooo good!! I absolutely loved both.

If convenience is important, I think the Shedd is a little far off from everything. If you do the Art Institute, you might be able to stop and grab some Chicago pizza. I say you "might" because you'll need all day to fully enjoy the Art Institute.

Have fun! I wish I could join you!! :) Chicago is such a beautiful city.

Added: my husband who lived there for six years in college and post college also suggested Magnificent Mile (Michigan Ave.), John Hancock Tower if the weather is nice to get a good view, the boat trip around the river for architecture, four-story Nordstrom, and Weber Grill for a good burger. I also want to add Chicago-style hotdog.

with no kids i would do the art institute. i love going there, and it is a bit closer to everything.
and as the others have mentioned the pizza!

No kids & only 4 hours; Art institute and John Hancock 96th floor. Dress warm & have fun. I've lived here my entire life and don't do as much as I should.