Dinan is certainly worth at least half a day. Nearby Lehon can be seen in less than an hour and it's well worth the drive or walk from Dinan. We took most of a day to drive around the estuary of the Rance. Lots of people like St Suliac. Thinking back, I seem to recall we liked the west side of the Rance estuary better than the east side. Once we passed St Suliac heading south, there was less of interest to us.
If you are book collectors, Becherel is a book town (mostly French books, of course) and it has a charm of its own even if you aren't shopping for books.
If you're interested in locks (canal locks), there are 11 at Hede, close together.
A quiet walk along the Ille-et-Rance canal can be nice in the evenings. Not very exciting for teenage boys, probably.