Hi Don, and welcome. If you could fill in your profile with your city/plus Ontario that would give us a better idea of your overall climate. Just the zone really doesn't tell us much eg. proximity to a lake can factor into both humidity and temperature where your cottage is.
You may have to use the "trial and error" method for a year before settling on something that works. Also, walk by some of the other cottages near yours, and see what other people have growing under their trees.
My first try would be something like Periwinkle
Vinca (Vinca minor 'Bowles's Variety') It also comes in white, and looks absolutely beautiful with spring flowering bulbs coming up through it. It says it's hardy to zone 4a, but if you dependably have a nice friendly mulch of leaves over it in the fall, that may help it a lot to survive the winters.
Another slight complication you're going to run into when first trying to establish some clumps of groundcover is tree roots. Not only do those trees drop a lot of leaves and needles everywhere, they also grow a large circle of roots around themselves. So be prepared to dig some good-sized holes for your starter plants, to give them a fighting chance against the trees. Fall planting can work if it's still warm enough there to try it. Again, be sure to mulch over the new plantings' root areas with a nice thick layer of leaves and give them a generous watering to get started.
