Nice towns/villages: St Ives, Porthleven, Padstow, Port Isaac, Marazion, Mousehole, Fowey.
Watergate Bay, Praa Sands or Hayle for miles of gorgeous flat beaches, Porth or Mawgan Porth for rockpools and beach "rivers" for kids to mess about in), Sennen Cove for surfing (and Watergate Bay for surfing too). We always stay in Watergate Bay, it's gorgeous though not much to do except the beach!)
History: St Michael's mount, Restormel Castle, Launceston Castle, Lydford Castle, Tintagel Castle, Halligye Fogou (not for the claustrophobic), Carn Euny or Chysauster if you like extremely calm peaceful stone-age villages!
Places NOT to go (disclaimer, in my opinion): Land's End (absolute tourist trap full of tat, extortionate prices - go to Lizard Point instead if you want to see the ends of the earth, it's just as breathtaking and much less tacky) and Penzance (strangely depressed and depressing town - very disappointing as I was expecting another St Ives) and Truro is just a bog-standard town, could be anywhere in the UK.