My first recommendation to everyone is the Hancock Building. If you want, the best way is to go to the lounge on the 96th floor, it's above the 95th floor restaurant, and you'll get beautiful views of the city! Instead of paying the money for the Sears Tower, just spend the money on a soda at the lounge and get a great view!!
I also agree that Navy Pier is a good place to spend some time. They have a ton of shops and restaurants, plus a few rides, included the ferris wheel ($6/person for about a 3-4 minute ride). There are various boat rides to take as well, a beer garden at the end (pretty good food there), live entertainment, and like the PP said, fireworks on Saturdays, at I believe 9:30 p.m.
The museums are great too, but there's just so much to see, that you won't get to it all. And it would be a shame to spend so much money (they're expensive, but check out that City Pass too) and then not get to see everything. The Shedd is really good, and closer to the downtown, along with the Field Museum. The Museum of Science and Industry is much further, about 40 blocks south.
Check out the city's website too, with it being Labor Day weekend, you will want to check out the hours of things, plus any special events that may be going on.
www.cityofchicago.org
Have fun!!!
ETA: I forgot, the steakhouses here can actually be quite casual too. I've never been to Shula's, but we've gone to the Chop House several times and Harry Caray's as well, and I've gone in a dress or in jeans and a nice shirt. Being a big city, they realize that the steakhouses are pretty well known, and a lot of people who come to visit don't want to get really dressed up, so it's never been a big deal. But if in doubt, just give them a call to make sure, or check their individual sites, it may say it right on there. BTW - the two I mentioned are my absolute favorites!!!!!