Yeah, we've been NT members for at least 6-7 years. There are 3 or 4 big, really good NT properties within half an hours drive and a number of other ones slightly further afield. We've been to Cliveden probably 30 times and it is NT and fantastic any time of year with huge house, lots of land, woods, etc and restaurants, orangery, etc. Hughenden Manor in High Wycombe is similar, house to tour and lots of grounds. Waddesdon Manor is a massive pile with lots of great gardens and grounds.
Someone mentioned conservation. The entire West Wycombe village is owned by NT, people rent the cottages, the pubs and shops are all rented from NT and all the buildings look like they haven't been touched since 18th century. West Wycombe Park is another house and property although only open in Summer. If you go a little further afield you can Osterley Park, Ham House, Basildon Park. Free entry, free parking, a really good deal and a good organization to be part of as they preserve so much that might otherwise be neglected or taken over by some wealthy landowner type.