Depends on where you are staying but Cornwall is easy to cover because everything is so relatively close to hand. Anyway, places to recommend:
Fowey - lovely little town to walk around with good cafes/restaurants etc.
Trevarno Gardens
St Ives - a must - great beaches, lovely to walk around; galleries; shops; restaurants; Porthminster beach cafe great!Asis Porthmeor cafe...
Sennen Cove - fabulous beach and excellent beachside restaurant/cafe
Lanhydrock House - old NT house and grounds - geared up for families
Polzeath beach and Daymer Bay (not far from Rock)
Godrevy and Gwithian (seals basking at Godrevy in summer) Near Hayle
Gweek Seal Sanctuary
Porthleven nice to walk around
Lizard Point -
Kynance Cove - excllent walk down to rocks etc...very dramtic. Cafe here does not take cards so cash needed!!!! We only realised once we walked down and could only scramble enough to buy two cups of coffee and a flapjack at lunchtime!!!
Roseland Peninsula - ver pretty!
Hope this helps - have a fab time!