I feel like the closest thing we've published at Paizo to an actual sandbox "adventure" isn't an adventure at all, but a book like "Sandpoint," which presents a region in great detail, offers lots of advice on how a campaign might develop, has lots of adventure hooks, map support for locations, and a lot of NPC stat blocks. Combine this with the numerous adventures we've set in the area (Burnt Offerings, Skinsaw Murders, and The Brinewall Legacy all come to mind off the top of my head) and there's a LOT of info out there for someone who wants to do a full-on sandbox campaign in the Sandpoint area.
You won't find this book, or other similar books, in our Adventure Path or Module lines though, since our adventures focus strongly on story and not regions. If you're eager to run a sandbox game, the best solution is to have at your disposal a large number of related adventures or a campaign setting style product.