There are a few English Heritage places
- dover castle, deal and walmer castle, Rochester castle, eyethorn castle and downs house (Charles darwins house).
Dover is a whole day whereas deal and walmer you could do both in a day and deal is a nice town.
Dreamland in Margate is worth a visit if you like old style theme parks. Margate old town is also nice.
There is a lighthouse at dungeoness that i believe to be English Heritage.
I have visited the powerstation before bit not sure if its still open to the public Absolutely awesome place but quite disconcerting to have to be checked for radiation levels upon leaving!
Dungeoness beach is fantastic.
Technically sussex but Rye Harbour and Rye town def worth a visit.
Broadstairs is lovely and you can walk along the seafront to ramsgate. Ramsgate tunnels is a lovely little exhibition/tea rooms comemmorating its use as air raid shelter and some sort of wartime secret railway.
Quex house in birchington is awesome. However be aware there are 100's if not 1000's of stuffed animals on display. If you hold on to the fact the place is 100yrs old over, and that genetic research is now being carried out on the bones you dont feel shit about it.
There is a great crazy golf therr too, non commercial and they have ducks!
Canterbury is amazing cathedral city. St Augustine's Abbey is another English Heritage place.
Whitstable - its a bit boden on sea but lovely (i live her, so biased!) Herne Bay is a nit more bucket and spade and good for younger kids.
Howletts zoo and pirt lympe are good. Also wildwood in herne bay. There is also wingham wildife park but im unsure about their ethos as a zoo.
Bedgebury pinetum near goudhurst as also worth a visit. It has go ape but if you have young kids there is an adventure trail with 10 playgrounds! The pinetum is stunning in spring.