Pendower if your around St Austell/Truro way.
Bit tricky to find, but beautiful and quiet. We don't live in Cornwall now, but when we are there we try and make a trip down to Pendower and have breakfast on the beach.
Poldhu near Mullion if you are south Cornwall. Bit smaller but parking is super easy and the cafe does the most amazing hot chocolates!!
Gwithian sand dunes are beaches are also very nice, used to get woken up 6am every morning in the summer when I was a kid to walk the dogs across them whilst it was cool enough! I complained at the time but would love this now!