Princes of the Apocalypse is a great sandbox adventure. It’s my favorite of the 5e adventure paths I’ve played or run. It does require a lot of work from the DM to run, however, partly because the adventure deliberately leaves a lot of gaps for you to fill in, and partly because the book itself has some real flaws with the way it presents information (a byproduct of WotC’s decision to combine what was intended to be two separate books into a single release late in the development process).
But if you’re willing to cope with that, then what you have in PotA is a campaign with two features that I think are definitive of sandbox play: unlike pretty much any other official WotC 5e content, a large portion of the book is optional side quests; and the various stages of the main adventure do not have to be explored in a predetermined order, meaning that unless the DM decides to adjust encounter difficulty on the basis of party level, parties can venture into very deadly places.