It depends on how many people you are, what you like doing and where in the country you want to go.
I could recommend places in the North East and Wales, as we've just come back / booked for these areas, but it depends what you like and how old the kids are. Haggerston, that someone mentioned above, has good activities for older children but our little one was too young for the formal activties.
If it helps, ParkDean are cheaper than Haven and have fewer facilities but will have the essentials, a pool etc.