Traveling east of Gananoque toward the Gananoque Narrows,
East to the International Bridge
After the Navy Islands, you can either go north of Ash Island or south. Either way you'll end up by Georgina and Constance islands and under the Thousand Islands International Bridge. If you stop at Georgina, you can take the trail around the island and see some of the bridge from the ground. It's an impressive piece of engineering.
Continuing east, you'll pass the park on Hill Island and once you pass Club Island, you'll have the quaint village of Rockport on your left. It's a pretty village and a good place to get gas.
From here, you can either head south toward Alexandria Bay and Boldt Castle on Heart Island (where you will need to clear US Customs). If you continue further east rather than going south to the US side, you'll have the choice of two routes. One takes you east on the north side of Grenadier Island and the other east by south route (the Canadian Middle Channel). You'll make your way to Adelaide, the most eastern National Park island in the Thousand Islands. Just east of that you'll see Singer Castle on Dark Island towering over the river. This attraction is well worth the visit. However, remember it is on the US side so you will have had to clear US Customs earlier to dock here and take the tour. (You can also take a tour boat out of Alexandria Bay, NY with Uncle Sam Boat Tours if you want to save time).