The historic Thames Valley and Chiltern Hills lie so close to London that you can easily reach them by car, train, or Green Line coach. In fact, you can explore this area during the day and return to London in time to see a West End show.
The second-most visited historic site in England is Windsor Castle, 34km (21 miles) west of London. If you base yourself in Windsor, you can spend another day exploring some of the sights on its periphery, including Eton (which adjoins Windsor), Runnymede, and Windsor Great Park.
If your visit coincides with the spring social sporting season, you can head to Ascot or Henley-on-Thames for the famous social sporting events: Ascot and the Royal Regatta.
Some great historic homes and gardens in the area include Woburn Abbey, Hatfield House, Hughenden Manor, Mapledurham House, and Wellington Ducal Estate. If your time is severely limited, the two most important country mansions to visit are Woburn Abbey and Hatfield House. Woburn Abbey could consume an entire day, whereas you can visit Hatfield in a half-day.
It's not just the historic homes that make the Home Counties intriguing to visit; the land of river valleys and gentle hills makes for wonderful drives. The beech-clad Chilterns are at their most beautiful in spring and autumn. This 64km (40-mile) chalk ridge extends in an arc from the Thames Valley to the old Roman city of St. Albans in Hertfordshire. The whole region is popular for boating holidays on its 322km (200-mile) network of canals.
Oxfordshire is a land of great mansions, old churches of widely varying architectural styles, and rolling farmland. Certainly your main reason for visiting Oxfordshire is to explore the university city of Oxford, about an hour's ride from London by car or train. It's not a good day trip, though, as it has too much to see and do. Plan to spend the night; the next morning you can visit Blenheim Palace, England's answer to Versailles.