Here’s a link to a useful leaflet detailing walks and places to visit along the Tarka Trail train.
On a nice day, get off at any station enroute and do one of the walks described in the walking book at the flat. The ticket is ‘hop-on,hop-off’ so you could even do two walks (or one walk and one pub visit??) in a day. On a grey day, why not go the whole way to Exeter and enjoy the cathedral, quayside and shops – including John Lewis? If you do want to get off at a small station, make sure you get one that stops everywhere though – not all do.