The first thing you need to let us know is how long you think you can dally on the driving trip. How much time you have for wandering will change priorities and eliminate or include certain options.
Next is to let us know whether you set higher priorities on scenery or history or museums or events or amusement parks or whatever.
I’d have thought that you would have left the Trans-Canada Highway several miles west of (before) Regina taken Sasktchewan Route 39 down to US-52 and on to I-94. That would have been a pretty direct shot. (If you took that route–and I am not really suggesting it–the one place where there might be something interesting would be Jamestown, ND (where US-52 meets I-94). On the southwest corner of town is the small Frontier Village museum of modest interest except that it has a sub-museum dedicated to Louis L’Amour, who was born in Jamestown, (if anyone in your party reads westerns) and it is the home of a pair of white buffalo.)
OTOH, I have nothing againt Winnipeg and I have a few options to offer with that route.
Toward the end of your Canadian journey, you will approach the Canadian Shield, although you will not enter it unless you travel a bit past Winnipeg. I have seen some (Canadian) dopers refer to it as boring, but it is unlike any scenery from the Rockies or the Great Plains and I think it is worth a pass through it with its alternately dense and sparse forests, interrupted by rock outcroppings and blue ponds). If, instead of taking Route 75 or Routes 300 & 59 South from Winnipeg, you stay on the Trans-Canada for a few miles into Ontario, you can take Ontario Route 71 South a few miles East of Kenora. This route will let you cross the Canadian bottleneck where the (at that point) two lane Trans-Canada Highway is the only paved road that that allows traffic to travel between Manitoba and Ontario and then give you a good view of the Shield.
Route 71 takes you down to Fort Frances, ON and International Falls, MN where you can pick up US-53 heading South. About 100 miles (160 kilometers) South is the village of Virgina. 20 miles (32 kilometers) West of that on US-169 is the village of Chisolm and the Minnesota Museum of Mining. Somewhere in the Virginia/Chisolm/Hibbing area is what, at one time, was the world’s largest open pit mine, with several other mines that were only marginally smaller. While they are certainly scars on the land, the sheer extent of human labor is impressive.
From Virginia, you can take US-53 South to Independence, MN and follow State Route 33 down to I-35 that heads to Minneapolis or stay on US-53 into Duluth, MN and Superior, WI. Superior is flat and industrial (although it has a neat museum in the last surviving whaleback freighter, the S.S. Meteor), while Duluth, built on the ridge rising up from Lake Superior has a lot of 19th century houses with views of the lake. The breakwall and light at the entrance to the Superior harbor shows up on a lot of Great Lakes calendars and postcards. You can pick up I-35 to Minneapolis in Duluth where US-53 crosses it.
Minneapolis and/or St. Paul have Mississippi cruises. There are similar cruises on the St. Croix river to Wisconsin and back. However, I suspect that they do not start until June.
After leaving Minneapolis on I-94, it is pretty much a straight shot to Chicago. If you have heard of and have an interest in the Wisconsin Dells, they can be fun. OTOH, nothing there is free or cheap. (The dells are a stretch of the Wisconsin River that carves its way through sandstone cliffs, which can be quite beautiful. However, it is all private land (no parks) and you have to pay to take either a boat ride or a Duck (DUKW ride to see the cliffs. It became a vacation getaway place in the days before trans-continental travel and has built up with golf course and amusements in the intervening years.)
Give a shot to my questions in the first two paragraphs and you might spark someone to throw out more suggestions.