I've only stopped by the Art Institute during free hours once (I'm a member, so don't pay admission during regular hours), on the Fourth of July. It wasn't crowded at all, though obviously the big attraction was the Taste of Chicago right outside, followed by the fireworks.
But, although Chicago gets a fair number of tourists year round, next week is NOT during heavy tourist season, as you're between the end of the summer/fall season (it's in the 30s this week, although next week will be much warmer) and the beginning of shopping season. It's almost absurd how many people come from all over the midwest to spend a few days of vacation/christmas shopping.
Back to the AIC. The lion's share (pardon the pun) of the impressionist and old masters' works are off display. Don't expect to see much, but there will be a few of the more famous pieces, as well as portions of famous collections. It is my understanding that they are periodically rotating some of them, so I don't know what will be on display next week.
The good news is that I just received the member email a few minutes ago (viewable online
here), and there is a new exhibit opening this weekend -
The Divine Art: Four Centuries of European Tapestries, which should be good.
Ordinarily I would highly recommend the Shedd Aquarium, as it is one of my favorite places in the city. The oceanarium closed around 6-7 weeks ago for major reconstruction, however, and will remain closed until next summer. Shark Reef and Amazon Rising are very nice exhibits, and the rest of the aquarium has continued to improve, but without the oceanarium I'm not sure if it is worth attending to the exclusion of something else.
The Field Museum is almost always worth a stop, and the current major exhibit is on Aztec culture. Or you could stop by Adler Planetarium to see the telescope that McCain keeps complaining about Obama "getting federal money for his hometown" for.
I'll give some more thought to less obvious options.