Friends have had great holidays in Eurocamp campsites in Italy (Lake Garda) and France (Brittany and near La Rochelle). All were chosen because they were close to beaches. The favourite was lake Garda where they went canoeing on the lake. Italy has great food for children too. There was scope for the DC to have a degree of freedom on the campsite too.
Unfortunately it was out of our price range at the time, so we had UK holidays.
Our favourites were Northumberland where there were lots of castles, beaches and a boat trip to the Farne Islands.
Similar with North Wales - castles, collecting lots of shells on the beach, Llandudno is a classic seaside town with beach, pier and cable car up Great Orme.
The Isle of Mull had a 'challenge' on arrival. Lots of options including a trail around Tobermory, a stamp from a steam railway, name of a tree on a particular forest trail etc and a prize at the end.
Going abroad, can I second someone else's suggestion of Barcelona?
We've not been with DC but I always wanted to. It is my favourite place. There is sooo much to do there. There are beaches, amazing buildings, a park where you can go boating on the lake, a cable car across the harbour, Parc Guell is amazing. The exhibits at the Joan Miro gallery would appeal to children too. And the Spanish actually like children!
We had a great week in the Loire Valley. We visited Châteaux, one with Sleeping Beauty in the attic, an old Abbey with tombs, cave dwellings, Gardens, and French markets. We had DDog with us which limited what we could do, but there were options to cycle along the Loire and canoe too.
Our holidays have always been busy though. We can't do more than a day or two at the beach. Or rather, I can't! We have a mix of busy morning or afternoon with relaxing at the cottage in between. If they can have quiet time in between you get less overtiredness.