Indianapolis to Elkhart Lake Wisconsin...

Our favorite route for years, which helped avoid some of the worst (including what is often a bottleneck at the junction of I-65 & I-80) was to take exit #220 off I-65 (Rte 10), and head west to US 41. Turn north on 41 and take it to S.R.2 (which becomes S.R. 17 when you cross the Illinois border) - that will go west a bit, then take a rather sharp turn to the right - continue north on that road (it will turn into S.R.1 after 17 turns off to the west) until it turns into a 4-lane and becomes 394,I believe). That will take you to I-90/294 tollway - go west on that & stay on the tollway around Chicago. Eventually it will reconnect with I-94 in Lake County IL, and you can take that north to Milwaukee. (Not 100% sure of the route after that, as we only went to Elkhart Lake once.)

If you stay on 80 and go west of the Chicago 'burbs, it will be a long way around, and more 2-lane roads, but may involve less traffic.