Killarney Provincial Park - often written as the Jewel of the Ontario Park System is arguably one of the most scenic landscapes in the Province. If you stick to your stated route of North Bay to Sault Ste. Marie it is not at all far off the route - and to miss it is to miss a lot. It would be southwest of Sudbury by about an hour and a half drive. The small campground at George Lake is as picturesque as you'll get anywhere - but you need to be aware that it does not have any hydro and is not big rig friendly. It is also incredibly popular and reservations made well in advance of your trip are advised. The town is as pretty as the park - and the local fish and chips have been written up in national magazines.
Check out the website: http://www.ontarioparks.com/park/killarney
Have a great trip.
Jay