A day in Falmouth is good - the Maritime Museum is great and the 'vibe' in the town is much less hectic than St Ives, the seagulls are friendlier too.
Gardens of Heligan are SO worth a visit.
For more activity based day there's a new water park at Retallack which looks amazing (not been yet but def on the list for this summer!)
Pendennis Castle at Falmouth - check online (English heritage) before you go as they have jousting and medieval days during the summer.
Cornwalls' Crealys (rebranded as Camel Creek) is a fun theme park.
Otherwise just 'hanging out' on the beach is one of the best ways to spend a Cornish day - just about as far from hectic London life as you can get! Bliss.