Having just come back from a trip to Landmark Trust's 'Gothic Temple' in Stowe I can absolutely recommend it. Staying in a temple in the middle of dozens of acres of National Trust grounds is fantastic. The temple has a triangular footprint wrapped around the central circular (domed) space with turrets at each corner of the triange. One turret is the stair tower, the other two are bathroom and kitchen (ground floor) and double bedrooms (first floor).

The nearest pub (The Queens Head) is a bit of a trek. The walkable route involves the back gate to the grounds being open; this is supposedly Wednesday to Sunday but we found it closed on Sunday. There is a small wall to climb down and a field to trek across which we didn't find too much trouble. The food in the pub is definitely worth the journey, though, although unfortunately they get fully booked up on a Sunday.

The grounds of Stowe themseves can also occupy quite a few hours. There are a number of follies and temples in the gardens, all positioned for views and generally not as interesting once you actually get near to them. Other sites nearby:

The Rothschilds place (can't remember the name of it), which was extremely decadent and interesting mainly for Baron von Rothschilds general lack of taste.
Silverstone: just about within earshot, but didn't go any closer.

For booking the Gothic Temple contact The Landmark Trust.