I just flew with my 20 month old son over Labor Day! Our flight was only an hour, but here's what I did:
1. I packed a backback with my carry on stuff and snacks/toys/portable DVD player. The portable DVD player was a life saver and my son loved watching the Baby Einsteins. I also went to the library and checked out some DVD's and some new books he's never seen. Another hit, the mini-etch-a sketch, he loved playing with that. I had a sippy cup of milk for the take off and didn't offer it until we were getting ready to take off.
2. I put my son in an umbrella stroller and strolled around the airport until I reached the gate. Once you pre-board, you can gate check your stroller and it will be waiting for you as you get off the plane at your destination.
As far as the carseat goes, if you paid for a seat for him, the car seat probably wouldn't be a bad idea, that way he'll think he's in the car. If he's a lap child, and if the flight is full, you lugged the car seat around the entire airport, plus your son, plus your carry-on and it's a BIG hassle.
Also, when I packed, I just packed 1 suitcase for my son and me vs. 2 suit cases, it was much easier to check.
Another thing, we left Midway just fine, but when we were coming back to Chicago, the airport I was leaving from asked for my son's shot record and/or birth certificate. I've never been asked for these documents and I've flown with my son over 4 times in the past 5 months. The agent said that he would have to charge me a ticket if I could not produce these documents to verify his age. I was furious b/c I was unaware of this requirement. Apparenlty if you have a lap child (one where you don't buy a ticket b/c he's under 2), and he looks close to 2, you have to verify his age. So something to consider if you don't have a ticket for your son, you might want to bring a copy of his birth certificate.
Good luck and have a safe trip!