Congratulations on your son's upcoming wedding! Just left downstate Illinois after 34 years, only 5 as a handgun owner, but I spent a lot of time researching the laws around Chicago due to family in the area. I am not a lawyer and these are only my (hopefully accurate) opinions after a lot of reading.
The definitive handout from the State Police is a good place to start.
Out of state residents do not need FOID cards to have a firearm in their possession. The card is issued by the Illinois State Police to IL residents to purchase firearms, ammunition, and in order to possess any type of firearm.
Downstate Illinois is pretty similar in that the state law and Wildlife Code are the main restrictions on firearms. If you are on your own property or place of business, carry concealed and have the firearm loaded and ready. Anywhere else, the firearm must be unloaded, cased and latched, or in a state where it cannot be operated (lock thru the breech, field stripped, etc). The safest way to travel in downstate Illinois is to have it in a case, unloaded, not accessible to the driver. However, there are many ways to be in compliance using the glovebox, center console, etc, as long as it is not loaded. In my opinion this is not looked upon favorably by law enforcement if you are stopped in traffic, but thankfully I never ran into that issue. Did carry plenty of unloaded pistols locked in the trunk without issue, but my arms weren't long enough to reach if I needed it
The issue with getting into Chicago and the suburbs is the town's ordinances are not superceded by the state laws. They can, and do, have ridiculous levels of detail....some say no magazines with more than 10 rounds, some say no types of ammo, some say no handguns entirely. Although I'm not a lawyer, an ordinance violation seems to be a lesser crime than a criminal matter, but the penalties can still be quite severe, including arrest and large fines. Plus you'd have to fly back to be in court...as someone from out of state you may not be intimately aware of the borders of each town...50 feet one direction may land you outside of Oak Park and into someplace with a completely different set of laws.
There were a few times I wanted to do some target shooting in the suburbs, but reading through all the different ordinances, and hoping that the law enforcement agencies would know these laws and enforce them correctly, I decided to leave the guns at home.
If you are going directly from the airport to the hotel in Oak Park and have the weapon unloaded and cased during transport, have it in the hotel room the rest of the time, then the only thing I'd advise you to double check is the Oak Park law since you will be staying overnight. I would not drive around Oak Park or other suburbs with it in the car even if you're just doing a post-reception Taco Bell run. There are areas of the state laws that allow for 'travel' through an area with a firearm that might otherwise be against an ordinance, but since you are staying overnight it may not be valid, as Ladder13 mentioned.
A S&W revolver with a medium barrel length and factory ammo might be just the ticket...that way you won't have to worry about magazine capacity, prohibited manufacturers, banned ammo, etc. All those things that they spell out in the ordinance.
A good folding knife with lockback mechanism that has a blade at 2" and a decent sized flashlight in the car are pretty good protection items as well. And yes, the knife laws are as messed up as gun laws...state law says something like 2.5" max, Chicago proper is 2", etc. But I think you'd have a lot easier time carrying a 2" folder anywhere in the city than a handgun. I always wore mine with a pocket clip so it was noticeable but not obtrusive.
Good luck!
