If there are any gardening fanatics in the family, you really ought to visit the Royal Horticultural Society's garden at Wisley. They're only about a mile from the M25/A3 junction. It costs �8 to get in but it's a good day out and there's a licensed restaurant.
Website here:
http://www.rhs.org.uk/WhatsOn/Gardens/wisley/
Review of a day trip to Wisley here:
http://www.timeout.com/london/features/1731/5. html
Alternatively, the attractive town of Cobham is only a couple of miles down the road from Junction 10. If you like (up market) Italian restaurants, La Capanna has good reviews. (�16.95, plus 12.5% service charge for a 2 course lunch). Website here:
http://www.lacapanna.co.uk/
Reviews here:
http://www.restaurant-guide.com/capanna-la.htm
If you'd prefer a French-based menu, The Plough might be worth a try:
http://www.themobilefoodguide.com/select/info8 090.php
Chris