If you (and your kids) like boats, history etc, we recommend Chatham Docks.
It's only about 40 minutes away, the tickets last a year and it's massive. You will definitely want to go back. They have submarines and war shops, lots of old dock buildings, interactive displays and various special events. They have special activities for kids (we had a really good science display which our son loved during half term).
It's such a big site that it doesn't feel that busy and the people who work there seem to love the place and often have a connection to it.
It's one of the best weekends we have had for a while. You could easily spent a weekend there.http://www.thedockyard.co.uk