The Whiskey Island Trail is a pretty flat trail that takes you along the old canal and onto Whiskey Island. The north portion of the trail is very swampy and should be avoided as it is often impassable. Start at the south end of the trail and make it an out and back instead. Follow the purple markers.
This route can be a good extension to the 10k loop if you are looking to add additional mileage. On the island along the river is the largest tree in the park which is nearly 6' in diameter, a very cool thing to check out if you have the time.