That's a good idea. All the traffic will be going the other way, in both directions!
If you want the bleak but dramatic North Coast, with towering cliffs and pounding seas, try Port Isaac (home of Doc Martin), Boscastle (near enough recovered from the floods), Tintagel (wonderful, ancient castle with spectacular views and walks). You can visit Rock and Padstow, though I don't recommend staying there, or going for long, it is too expensive.
The South Coast is mild and much kinder. I recommend Falmouth, Fowey, Looe, Pentewan and Mevagissey. Within easy reach are various castles, Lost Gardens of Heligan, and you can't miss the Eden Project.
Further down into Cornwall, there are Penzance, Marazion, St Ives, St Michael's Mount, the Cliff Theatre at Minack, and the Space Museum at Goonhilly. Truro has a good shopping centre, and the youngest Cathedral in England.
I don't recommend Newquay (tacky) unless you want surfing or a hectic nightlife, or Land's End (not worth the money).
Launceston, the Gateway to Cornwall, is a quaint old town worth a visit if by-passing on the A30 dual carriageway, with a Norman Castle, and good coffee shops.