Blenheim Palace The Palace is home to the Duke and Duchess of Marlborough and birthplace of Sir Winston Churchill and open to the public year round. This magnificent house, lake, park and gardens (designed by Capability Brown) is at Woodstock, a few miles from Oxford and about half an hour from the cottage. There is so much to see and do that you could happily spend a whole day here and make the hefty entrance (true for 2016) charge worthwhile - £24.90/adult or £19.90 for seniors, £13.90 for children, although this does convert online into an annual pass as well so you don't have to do it all in a day. If you have children under about 10 you would probably only want to skip going round the Palace as there are lots of things to do in the grounds - ride on a narrow gauge railway, get lost in a maze, walk through the butterfly house, play in the adventure playground as well as walk around the beautiful gardens and the lake. Note that Blenheim is part of a group called the Historic Houses Association. If you join the HHA you get free entrance but of course the HHA has an annual membership so it will only be worth your while if you are planning on seeing other HHA properties (eg Kelmscott Manor, Highclere Castle, Berkeley Castle).
Kenilworth Castle Now a ruin, this was the home of Robert Dudley, favourite of Elizabeth I and the Elizabethan Gardens, lost for 400 years, have recently been authentically recreated.
Warwick Castle This is a very popular tourist attraction in the town of Warwick and some visitors complain to me that it has been "Disneyfied" and they have even thought that it was built specially for the tourists! In fact, it is a genuine, extremely well preserved medieval castle, albeit altered over the centuries. Originally, in the time of William the Conquerer (he had it built in 1068) it was a wooden construction, later rebuilt in stone and used as a fortification until the early 17th Century. It was then converted into a country house by a Greville and this family later became the Earls of Warwick and owned the castle from 1759 until 1978 when they sold it to Tussauds, who own the famous Wax Museum in London. Amongst the many things to do there (see the website) you can go back in time, peeping into the private apartments to see a reconstruction of the preparations for a Royal Weekend party of 1898 when the Prince of Wales (later Edward VII) came to stay at the castle, with the very convincing wax figures standing in for the house party characters.
Sudeley Castle at Winchcombe. There is lovely drive on the back roads from Chipping Campden to Winchcombe if you aren't in a hurry. Most people visit here for the gardens but you can take a tour of the private apartments (check about children under 12 as I am not sure that they can do this). There is an adventure playground and nature trail also. A member of the HHA with 50% discount for members.
Berkeley Castle, ( free entry to HHA members) open until end of October, Sunday to Wednesday a little further to drive in the South of the Cotswolds, is the oldest castle in Britain to be lived in by the same family for nearly 900 years. The murder of Edward II is thought to have taken place here in 1327. A Norman Fortress, 'converted' in medieval times to become a family home with beautiful Elizabethan gardens as well as a butterfly house. The tour of the castle is guided and included in the admission price but I think you can also wander on your own. It was used as a set in the filming of the period drama 'Wolf Hall'. While you are in the area, you could also visit the Edward Jenner Museum and medieval church of St Mary next door. You could spend half a day here and then the other half visiting the Slimbridge Wetlands.
Not exactly local as it is about an hour and half to 2 hours (straightforward, but sometimes busy road) drive but fans of the popular costume drama, 'Downton Abbey' might like to visit the beautiful setting for the series Highclere Castle Do note that tickets must be purchased in advance. Now is the time to think about 2017. The only way of going without advance booking is to join the Historic Houses Association (HHA). It is an annual fee but may be worth it if you plan to visit others that are covered by the Pass e.g. Blenheim Palace, Seizencote House and gardens near Moreton in Marsh, Kelmscott Manor, home of William Morris to name but a few within a reasonable distance. Another member of HHA, a little further away, is Berkeley Castle ( see above) At Highclere, there are some events on for charity in early December for which you can buy tickets and pre-booking for this time is open at the end of September. Dates and tickets for Spring 2017 will be released on their website from 1st November. Summer 2017 opening goes on sale in the first week of February. It is very popular, a bit like booking for a pop concert - you have to get on the website pronto and be the first ones to get tickets as they are limited ! Full details are on their website here