There's some nice walks along the north Kent coast (Saxon Way?) including Reculver Castle. Canterbury is good, you can also try Rye, Tenterden (including Bodiam Castle and the Kent & East Sussex Railway), Leeds Castle, Dover Castle (with a great underground wartime museum), Sissinghurst gardens near Cranbrook (bit further away, depends on age/interests of family). Whitstable has become a desirable town recently after years of decline - the famous oysters have been a catalyst I'm sure).
Have a great time anyway!