Rousham Garden is close to junction 9 on the M40. The privately owned house is only open for pre-booked groups but the gardens are open during the summer and are well worth visiting though no dogs or children under 15 are allowed. We went on a sunny day in late August and felt it a delightfully peaceful and uncrowded garden. Entrance cost £5 (from a ticket machine) and although there are no refreshments available you can bring a picnic.
The gardens were designed by William Kent in the 16th century and are largely unchanged. They are mainly in two areas, one close to the house and church and another larger, less cultivated area.
The area closer to the buildings is fairly sheltered, divided into smaller gardens, with flower beds, a pigeon house and fruit trees. The larger area has several pleasant walks and views with various ponds and cascades. We wandered around for 2 or 3 hours and could have spent longer if we had explored the furthest areas. It is worth checking on the website www.rousham.org to make sure it is open on the day you plan to visit.