That's a long drive so I'm guessing you won't want to stray too far from the 401, but Prince Edward Counties has Sandbanks Provincial Park, Lake on the Mountain Provincial Park, and lots of nice scenic shoreline on the southeast and southwest corners of the island.
On the 1000 Islands Parkway, there are lots of little parking spots along the way. I think the one across from the Landon Bay Campground is nice, or you can go into the campground and take a short hike up to a rocky lookout above the water. Admission is by donation. Further down the Parkway you can cross the bridge to Hill Island and take a trip up the 1000 Islands Skydeck. There are also parking lots at Mallorytown Landing and Brown's Bay Park, but I think you have to pay for parking at both (for sure at Brown's Bay but they have picnic tables).
In Iroquois, you can head down to the lock and check out the large ships being dropped or raised (a few feet). You can check arrival times at the Seaway website:
www.greatlakes-seaway.com/R2/jsp/R2.jsp…
Futher down the road, you can go in the entrance ot Upper Canada Village. Stay to the right and enjoy the free parking and river views from Crysler Farm Battlefield.
A bit futher east is the Long Sault Parkway. It is a series of hilltops turned into islands when the region was flooded to make the Seaway. The islands are connected by a series of causeways and bridges and there are places to picnic or swim. There is a parking fee, but plenty of picnic tables and a few beaches. There are also a few places where you can see the old roads going under the water.
Edited: 12 July 2013, 17:37 
