Eden Project (if you have the cash and its semi inside - I think circa £70 for a family of four).
Maritime Museum in Falmouth (there's a Rick Stein's Fish and Chip shop in the square and a cracking good Pizza Express with views over the Marina as well.
St Michaels Mount with ice cream in Newlyn at Jelberts (old fashioned Cornish) and a walk around the fishing boats.
There are some old steam trains.
Fish and Chips at Indian Queens/St Columb, the Port and Starboard (brilliant)
There may well be some surfing and kites at Watergate, Fistral Bay and Perranporth.......
Cornish Pirates Rugby in Penzance (good and welcoming for kids).