http://www.plusbus.info/I use trains to get out & about in Devon & Cornwall and often use buses to complete my journey.
On the buses I'd usually get a point to point fare except in Plymouth where a day ticket is good value.
Day tickets can be a pain as you are tied to an operator unless you are in mid Cornwall where FirstBus & Truronian share ticketing - you can cover a lot of ground and don't have the BOJ problems.
PlusBus is even better for small areas as it (usually) covers all operators, add in a rail card (only way to get a discount on a bus) and with some planning you can really get around without worrying what colour a bus is.
Firstdayplymouth is ^3.20 but Plusbus is ^1.60 with a railcard and you can use any operators bus.
Some areas are so small or only have one operator that it's not worth it BUT....some are huge...Camborne Redruth in Cornwall is great....you can visit the mining areas from the Great Flat Load at Camborne out to the Poldice valley, where Julia Bradbury has just done a Railway Walk, and end the day at Portreath beach or Tehidy Country Park.
Best by far is Newton Abbot....^1.30 gets you unlimited travel to Shaldon, Babbacombe, Torquay, Paignton, Brixham, Kingswear. Buckfastleigh....I get up this way a lot to visit Paignton Zoo, Living Coasts, Kents Cavern & Dartmouth for cruises.
PlusBus sometimes won't issue through
Avantix▸ i.e. St ives/Redruth....you have to get a normal train
CDR▸ and a Redruth to Redruth PlusBus (which is a prohibited fare
)...similarly St Ives/Plymouth might need a BOJ before PlusBus will work.
I could get a CDR to Plymouth and a Ride Devon but the 'Ride' tickets don't discount with a rail card plus about as far as I can go in a day is Exmouth and still have time to do anything.