The Saco river is fun and popular. The East Branch River is off of the beaten path and a relaxing area. Just off of 16A is Town Hall Road. You will find Flat Rocks Conservation area with parking and access to the East Branch River. Below is a screenshot of the Maps which shows the location of Flat Rocks.
We like Flat Rocks because it is quiet and the dogs can go in the water. You could bring some chairs and hang out for a while or the day. It is sunny and shady.