Best in early or late season before the poison ivy gets too thick.
The Presumpscot River Trail starts behind Corsetti's in Westbrook. It proves a nice, long, flowy singletrack that skirts the Presumpscot River all the way to Falmouth. Once you reach Falmouth Road you can turn around and head back.
There are a few challenging uphills and a couple wooden jumps in the beginning, but it's mostly flat singletrack.