Threave Garden is Castle Douglas's best reason to stop. The estate mixes formal planting, glasshouses, woodland, wetland, and bird hides, so it is more than a pretty garden for an hour between drives.
The National Trust for Scotland runs the estate, and the nature reserve gives it a wild edge. Bring binoculars if you have them: ospreys, bats, waterfowl, and woodland birds are part of the appeal.