Grab a bite to eat and a pint and watch the foot ferry then when you are fed and watered, head over to the shared path that runs from Exeter to Dawlish and enjoy a mostly traffic free ride.
Topsham ferry run most weekends, and on four weekdays a week during spring to autumn. But it depends on the tide so it might not be working when you need it! Still, it's only a 4 or 5 mile detour to go round Countess Weir.