Kingston / Thousand Islands / Boldt Castle (bring your passport)
For more small-town charm, check out the historic towns on the Rideau Canal, such as Smith Falls, Merrickville, Manotic etc.
Of course, Ottawa / Gatineau is nice in the summer too.
Pelee Island (never tried it myself, but always wanted to)
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If you want to really escape, I would highly recommend a run across the border. When I lived in Ontario, my favorite weekend get-aways were:
* Cedar Point, Sandusky, Ohio (Roller Coaster capital of the world!)
* Hershey, Pennsylvania (Hersheypark amusement park, Hershey Chocolate World, Hershey Hotel and Spa, tours of the historic town of Hershey, outlet shopping...)
* Lake Placid in the Adirondack Mountains, New York (it's the Banff of the east)