I'd definitely recommend the Goodwood Revival, we never miss it, took a friend from the States last year and he was blown away.
Highclere is only open to the public a limited number of days each year so you may not be able to visit around the Revival time.
Warwick castle is great too.
If you really want to push the boat out I'd recommend a night at Amberley Castle, it's a real castle that's now a hotel. Not too far from Goodwood, amazing rooms and fantastic food, not cheap though.
