I think anywhere great is going to be busy, especially this year. And it depends on what sort of things your DC like doing.
Cornwall is busy for a reason. Great beaches, inland and coastal walks. You can go surfing and sailing and other watersports.
Similarly Devon. Little beaches in South Devon around Hope Cove and my favourite, Thurlstone beach. Posh Salcombe for shopping.
Wales is lovely. In North Wales there are walks in Snowdonia and some fantastic beaches. Llandudno is an old fashioned seaside resort with a cable car up great Orme. There are miniature railways and castles to look around in Conway and Caernarfon.
In South Wales we stayed near St David's, a pretty mini city. Lovely beaches in little coves and again coastal walks. There were places offering Canyoning but my DC were too young at the time.
Northumberland is relatively quiet. Bamburgh beach is stunning. The castle at Alnwick was the setting for some scenes in Harry Potter and there is a poison garden which you can take a tour around. You can go across the causeway to Lindisfarne and take a boat trip out to the Farne Islands to see seals and lots of birds. Hadrian's wall isn't too far away nor is Kielder forest for walks.
The Wye Valley? Canoeing around Symonds Yat - they drop you off up river and you paddle with the current, stop along the way for lunch at a riverside pub and then get picked up again down river. The countryside is 'crumpled' looking, there are little towns and as it is border country, run down castles.
And of course beautiful Scotland. We went to the Isle of Mull when DC were young and there was a so much for them to do. We planned to go back when they were older to get a boat to Fingal's Cave but never quite managed it.
Cromer and Norfolk? The Norfolk Broads on a barge? Brighton - beaches and lots of little shops?