If you feel particularly energetic, you can take your bike on the train to Okehampton, follow National Cycle Route 27 across the Meldon viaduct and then onto Tavistock, passing Lydford Castle and through Lydford Gorge,
Indeed you can, and then the Plym Valley trail from Tavistock to Plymouth is another option, including an impressive newly-built bridge high above the Walkham valley and several old regular stone bridges, with lots of fine views.
To do the Calstock Viaduct, you can get on the train from either Gunnislake or Calstock, That stretch of the Tamar is more a gorge than a mere valley, and the views are worth taking in (though sadly the train line misses those views except at Calstock). There's also a much lower but longer (iron) viaduct on that line, from the Bere peninsula across the estuary of the Tavy and Tamerton Lake into the Plymouth suburbs.
Those of us who live in this part of the country desperately want our railway back: Plymouth to Exeter via Tavistock and Okehampton (i.e. skirting Dartmoor to the West and North). That way, even if it's just a single line with speed restrictions, we're not completely cut off whenever today's line succumbs to the sea around Dawlish/Teignmouth. This would imply re-opening the Meldon Viaduct.