wow - so many great suggestions! Thanks everyone for taking the time to help - I really appreciate it! :)
Well see, it's a bit hard to find something we would all enjoy. My mother and I would enjoy going almost anywhere and walking around and taking the sites in.
My father however... is where it gets tough. He is famous for saying things such as "it's just a pile of old rocks" at the Acropolis, or "it's just a building" at the Eiffel Tower. He loves WWII, and ships/oceanside. He is.. shall I say stereotypical American. A bit larger and enjoys taking rides through places to see the sites from the comfort of his car. He seems pretty set on spending a few days in Portsmouth and driving through the coast. My objections to that is - I've been to Portsmouth, I don't feel there is a few days worth of activities. Driving the coast in late October might not be the best season and with the cars being a lot smaller than he's used to - an uncomfortable ride for him :(
We are visiting friend in Essex the first night - I suggested the next morning we drive to Oxford, spend a few days around there, see Blenheim Palace, drive through the Costwolds (looks gorgeous thanks for that suggestion!) and on our way back to Essex stop in at Windsor Castle.
I'd love to take him to Portsmouth, see Brighton, and Dover. But I sort of felt maybe that is a trip for warmer weather no? I could be totally wrong!
Looking forward to hearing everyone's thoughts :)