Dungeons & Dragons is only a set of rules. Many decisions of game design, which in a computer game would be done by a game developer, in Dungeons & Dragons are left to the DM to decide. Which means that a DM frequently has to deal with questions of good game design. One of those questions is whether to run a more linear, story-driven game, or more of a sandbox game.

Compared to a computer game, a sandbox is easier to create in a pen & paper roleplaying game. In a computer game, you need to create the whole world that can be visited in advance. In a pen & paper the details of any place can be created when the players are actually visiting it. You might have some campaign material describing a city in half a page or less, but once the players go and explore the city the interactive storytelling between DM and players creates more and more detail of that city. With a thin book of a campaign setting and a bunch of random encounter tables and random dungeons you could fill years of a sandbox campaign. Which might be fun to the people who like sandbox gameplay very much.

So after reading some good reviews (unfortunately I missed the more realistic review), I bought the 5th edition Dungeons & Dragons adventure Storm King's Thunder, which promised a great sandbox campaign from level 1 to 11. And after studying it in detail, I am very disappointed. Because the sandbox part is only one chapter in the book (admittedly a thick one), and is only supposed to last the time it takes to get from level 6 to level 7. And I don't really like the structure of the rest of the book (Warning, spoilers ahead!):

• While the low levels in 5E are generally fast, the chapter getting the players from level 1 to 5 is ridiculously thin. They basically explore an empty village in which goblins in groups of 1 or 2 are dispersed to get to level 2. Then they get attacked by orcs and make level 3. Then they visit one short dungeon to make level 4, and then they get attacked twice while traveling to make level 5.
• Level 5 gives an illusion of choice: It could happen in three different locations! But each of the locations has the same basic story, an attack by giants looking for something. And of course being attacked in this adventure gets you another level.
• Level 6 is the sandbox part. So while up to now the players gained a level every time they sneezed, they now play 60 pages of the book to get to level 7. Because the level gain is linked to ending the chapter, this is a sandbox that rewards players to explore as little as possible. If you want to permanently discourage players from sandbox style roleplaying, this is exactly the way to do it: "Here is a sandbox which you can explore to your heart's content, but you'll only gain a level if you stop doing so and follow the linear story again!".
• Level 7 is okay, a dungeon to get to level 8.
• Level 8 has actual choice, there are five different locations which you can visit to get to level 9. Unfortunately you can't visit more than one of them, so of the 63 pages describing the locations, the players will only ever see one fifth. And the players have no way of knowing what the five locations are about (other than the type of giant), so they might well not end up with the locations which would be most suitable to their preferences.
• Level 9 to 11 is a linear story. There aren't all that many encounters between two levels.

I believe the fundamental problem with this printed "sandbox" adventure is the need to be at a specific level for a location, because the locations come with combat encounters of that level. If Dungeons & Dragons didn't have levels, the adventure would probably work well enough with some tweaks. But as it is the players are rewarded for rushing through chapters, because it is the end of the chapter that gives the level-up reward.

I do think that one could build a great sandbox campaign out of Storm King's Thunder in combination with the Sword Coast Adventurer's Guide. But that would require ditching the chapter that gets the players to level 5, and using the sandbox part from level 1 to 7 (Including the attack on town part at the appropriate moment). The rest of the adventure could be reworked to require the players to visit all five giant locations instead just one of them. That would still be a more linear adventure though. The main work for all of it would be having to rework all the encounters as they happen to whatever level the players are at the time.

For my campaign I will first read some other adventures and see whether something less sandboxy isn't more suitable for my group.