Route 66 from Chicago to Los Angeles is one of my favorite road trips. I have done this trip from both directions and my favorite is from East to West. I just guided a group of South Africans in a Motor Home from Chicago to L.A. on Route 66 in early October. They had won a contest and the winners received a fully paid 10 day trip in a motorhome from Chicago to L.A. with a flight from Cape Town to Chicago and at the end from L.A. back to Cape Town. They were amazed at how much American history was a part of this iconic road. We only traveled about 150 to 200 miles per day and actually got off the freeways and traveled on the parts of the old road that still exist.
From downtown Chicago we could drive on most of the original route 66 road to Joliet, IL then we got on Interstate 55 and traveled to Pontiac where we were able to travel on a section of the old road through Ponitiac, IL There's a great Route 66 museum in Ponitiac. Old Route 66 is well marked all the way through Illinois, however, there's sections where you have to get back on Interstate 55. There is a great old bridge over the Mississippi River from Illinois to St. Louis, MO - the Chain of Rocks Bridge. It no longer has vehicular traffic, but you now can walk across the bridge. There's a great view of the St. Louis skyline in the distance and the Gateway Arch. One of the Route 66 icons in St. Louis is Ted Drews Custard shop. This is a must stop. Traveling through St. Louis you will want to drive on Interstate 44 westbound, however there's little sections of Route 66 that travel through the little local towns. These sections all through Missouri are also well marked and there's also little old gas stations, diners and motels that have remained as living museum pieces.
I would recommend you go to this web site: http://www.theroadwanderer.net/66Illinois/route66IL.htm
This web site has a description from Chicago to L.A. section by section and not only lists the things to see but there's some great pictures as well. I think you will enjoy reading and researching your trip. I would be happy to answer any questions you might have.
[ Edit: Sorry, no promos please. ]