We went to St Ives for the first time last year. Really loved Porthmeor for the waves, body boarding and sunsets. Loads of fun.
Carbis Bay, beautiful. I was surprised by how many different beaches we could walk to around St Ives, no wonder it's a popular place.
But I also like a quieter, calmer beach too. I can't really recommend any of those in Cornwall tho. I'd be interested to find them!