I rode there from Harvard this summer. Here's a link to my friend's blog entry with route: http://reallyboring.net/?p=28
Mostly quiet country roads (some unpaved) most of the way though we were forced onto some busier roads closer to Lake Geneva. Of course if your riding there from the city you would probably want a completely different route.
I used (from Elgin) this route:
If you don't like the crushed lime stone section between Ringwood and Richmond, use alternative roads (Ringwood Rd - Pioneer Rd - Solon Rd - Rt. 31) instead.
Have a good trip,
I did this ride yesterday- started on the North Shore Trail, then Green Bay/McClory to Millennium Trail to Prairie Trail to regular roads to get into Geneva. It was a nice ride (though super hot) but there is massive road construction just after the bridge you hit as you exit the Moraine Hills park- on Charles J Miller Road (west of S. River Road). It was a big mess that forced us to walk/carry our loaded bikes through maybe a quarter mile of sand/construction gunk and doesn't look like it'll be cleared up anytime soon. Road on the other side of the street was very tight with no shoulder. But other than that the ride was great! We skipped out on a few parts of the Millennium Trail that aren't direct and found the county roads around there to be okay. Not great, but okay. Drivers gave us a ton of room whenever they could but there was construction that created choke points that were tricky.
Rode to Harvard to take the Metra home today- some really nice hilly back roads on that 20 mile ride. Many intersections out that way have no street signs though so GPS or some navigation aid is critical.
On the way to Waterford, WI, I've gone as far as Genoa City at State line on the Prarie Path. From La Grange Park I took the Salt Creek Trail to the Illinois Prarie Path (IPP) to the Great Western Trail (Starting in Villa PK) back to the IPP Elgin Branch to the Fox River Trail to the Prarie Path. It's not as direct or fast as roads, but you are in shade most of the time and it's fairly flat (Except just north of Crystal Lake). From LGP it's 67 miles to the state line. The Fox River portion of the trail is beautiful.
Its been a few years so you may want to check on some of this but its a delightful ride. Here is a cue sheet to Lake Geneva from the Panera in downtown Wilmette.