I live in Cornwall, near Falmouth, and I'd recommend riding around the entire coastline - you're spoiled for choice with beautiful beaches, dramatic cliffs, pretty villages, ruined tin mines, fantastic back roads...The north coast is more dramatic than the south with bigger sandy beaches and bigger seas (which is why surfers love it). The south coast is more sheltered but equally picturesque. Avoid St Ives and Padstow if you don't like crowds, especially if it's sunny and a weekend. We love the beaches at Hayle/Gwithian and Perrenporth for long walks at low tide.
A great ride would be to follow the coast from, say, Falmouth down to Marazion/Penzance/Newlyn/Mousehole/Porthcurno/Sennen Cove then continue around the western tip up to Bottalack/Zennor/St Ives/Hayle/Gwithian/Portreath/Porthtown/St Agnes/Perrenporth etc etc. Land's End is very underwhelming but if you park up and follow the coast path to the left of the hotel the views are stunning.
The Roseland Peninsula is really pretty and fairly quiet. Ride towards Trellisick Gardens (NT) and get the King Harry Ferry across the River Fal with your bike then ride to St Just-in-Roseland (lovely creekside church), St Mawes and Portscaotho. The Hidden Hut by Porthcurick Beach just outside Portscatho is worth a detour (10 minute coastal walk from the carpark) - food is great and you can just turn up at lunchtime, order for food and sit on the grass looking out to sea!
A great way to ride into Cornwall is on the A39 Atlantic Highway from Minehead in Somerset. Popular with bikers as it's a fantastic road with great views. Ride it to Bude then maybe cut inland and ride across Dartmoor - stunning countryside, lovely roads - Tavistock to Princetown, Two Bridges, Dartmeet, Widecombe in the Moor, Haytor etc. The Two Bridges Hotel is a decent place to stop for lunch and even stay over if you fancy it.
From Tavistock on Dartmoor it's about a 2 hour ride to Falmouth in Cornwall and the A30 is a great road these days with some very long and sweeping sections where you can see for miles and open the throttle up (just watch for the speed camera vans!)
I could go on, but I don't think you'll have enough time!