Some Very Basic Outlines to Get You Started
Welcome aboard the RoadTrip America Forums!
I understand how daunting trying to plan a cross-continent journey can be, However, Donna is correct in saying that we can't really help you much until you've chosen a 'backbone' for your drive. So, that's the (very limited) help you need at the moment - what are your basic options. The first, and most direct, is to continue north from the Grand Canyon up US-163/US-191 through Monument Valley and past Arches National Park to I-70 across the Colorado Rockies to Denver. Then you would take I-76 to I-80 and follow that (and the old Oregon Trail) east to Chicago, then up through Michigan on I-94/I-69 to Sarnia and home.
An alternative route that stays a bit south of that might have you continuing east from the Canyon on I-40 past Petrified Forest National Park, with possible short detours to Santa Fe and/or Taos then on to Oklahoma City before picking up I-44 to St. Louis, a city with much to recommend it. You could then either head up to Chicago, etc. as above or take I-70 through Dayton and its aeronautical history to Columbus and up around Lake Erie and through Niagara Falls to home. That 'southern' route adds all of about 150 miles to your journey.
Another alternative would be to head a little west(!) and north from the Canyon on US-89 through the Grand Staircase-Escalante National Monument to Zion and Bryce Canyon National Parks, Salt Lake City and all the way up to Yellowstone National park before turning northeast and basically following I-94 through Montana and North Dakota past Theodore Roosevelt National Park. You would then take US-10/MN-210 across the 'Land of 10,000 Lakes' to Duluth and US-2 across the UP of Michigan to Sault Ste Marie where you would cross the border and return home on the Trans-Canada Highway. This route would add a good bit of driving, about 600 miles, but is still within reason.
Only you can pick between these possibilities based on what you want to see, how much time you have and how much driving you want to do. So now follow Donna's advice. Get out some maps. See where these routes take you and what's along them. Once you find one that's to your liking we can offer more specifics of smaller, but still interesting and enjoyable, sites to visit.
AZBuck