There's a number of ways to get to the Druids Cache. A suggested starting point with ample parking is at N 46 49.536' W 113 56.304'. Not the closest possible approach, but one that provides a pleasant walk through the forest. If you use this parking lot, be sure to pick up a brochure and map to the local trails. (Not all of the secondary ones are on the map, however.)
The trailhead closest to the parking area which leads near the cache is for the secondary trail P139, at N 46 49.459' W 113 56.395'.
Enjoy the hunt and the recreation area.
Note: The recreation area is open during daylight hours only, so plan accordingly.