We've only driven it once from the Sheboygan area, so take this advice with that in mind. (There are many here with much more driving experience than me.) We took 65 down, but just to change things up a bit we took 57 home. My experience was that 65 was the more fun of the two routes, because there are more cities to see; I also felt like on 57, you are in Illinois for such a long time that it seems like you aren't making any progress at all. But, 57 is a little more relaxed, you'll avoid Indy and Louisville, and you won't hit nearly as much truck traffic on 57 either. As others have said, you'll have to deal with Nashville (and Atlanta) either way.
I've seen a few suggestions of avoiding Chicago. Personally I had no problems with Chicago on the way down, but we left home at 3:30 am on a Saturday morning, and hit Chicago around 6:30ish am. So I'm sure that helped a ton. On the way home, we ran into some Chicago traffic, but it flowed nicely and didn't slow us down at all.
Not sure where you are planning on stopping for the night, but we ended up staying at the Drury Inn in Morrow, GA, which is just south of Atlanta. We got there around 6:00 pm, left around 8:30 am, and really took our time getting to Orlando since it is only about a six hour drive from there.
We're making the drive again at the end of August, and based on a few suggestions I saw here we're leaning heavily towards heading towards Columbia, SC, staying the night in that area, and then heading to Orlando along the east coast. It'll add an hour to our drive, but we'll avoid Nashville and Atlanta (where we lost the most amount of time due to traffic and construction) and it'll give us a chance to see more of the country. I can't wait!