The historic city of Kingston is the centre for Thousand Islands activities although it is west of that stretch of the St. Lawrence River. The town of Gananoque, just to the east, anchors a scenic stretch of secondary highway following the river's flow. You can find boat excursions, Fort Henry, etc. Google.
I wonder how much War of 1812 you will see in 2013. Last year's government-induced interest has cooled down somewhat, and just as well too.
The nation's capital, Ottawa, is the bigger eastern Ontario centre with plenty of attractions and its own historic river. It's an hour or two drive away from the St. Lawrence. Montreal completes the triangle.

Depending on your starting point, I'd strongly recommend taking highway #2, the Loyalist Parkway and the 1000 Islands Parkway to stay as much along the river as you can. It's a great drive and I haven't done it in too long.

This July (12th-14th) at Upper Canada Village in Morrisburg, the War of 1812 re-enactment of the Battle of Crysler's Farm is an "all-in" affair being given special attention due to this being the bicentennial year for that battle. Here's a link to their page.

http://bit.ly/Battle_of_Cryslers_Farm_bicentennial

If you can't make that particular event and are interested in catching one elsewhere, here's a link to other events

http://bit.ly/War_of_1812_events_list

The ones in red are other "all-in" events that'll also be getting a larger-than-normal turn-out by reenactors.

HTH
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