We had a good time on the steam train from Tenterden, it goes to Bodiam Castle which is nice as well - we didn't actually go in the castle this time as we've been before but still enjoyed walking round the outside and looking at the ducks and huge fish in the moat before going back on the train.
Depending on what part of Kent you end up in, another nice place to go is Bedgebury Pinetum, just over in East Sussex, good walks and adventure playgrounds. I've also heard that Woodchurch Rare Breeds Farm is a good day out, although I've not been for about 20 years!
Afraid I've no idea about campsites. Whereabouts is the wedding?
Jiglet, Mar'09 & Bell, Jan'12