…Yeah, okay, that title doesn’t make much sense. >.> But we’ll roll with it!
My Storm King’s Thunder campaign is rolling along nicely, going on something like two years now. SKT was hailed in reviews as being a terrific sandboxey adventure, which it kinda-sorta is, and kinda-sorta isn’t , but we are now in the “post Eye-Of-the-All-Father” stage of the campaign, which very much isn’t a sandbox, as written. I want to avoid spoilers, but I will say that if you’re familiar with SKT, you will probably know what I mean when I say that from Chapter Five forward, it’s pretty much a railroady race to the end, with scripted cutscenes for NPCs to have big moments baked in and everything.
Which, y’know, makes a certain amount of sense. You can’t really write “a campaign in a book” like this and have the ending make any kind of sense, if you don’t collapse all the probability waveforms down into a single cohesive storyline. But running a railroad game is fundamentally opposed to my Gamemastering credo, specifically items #7 and #9. So how to resolve this?
Actually, the answer is also in the Credo, specifically item #11. I’m tossing in a bunch of potential side-quests of my own design, most if not all of which are optional. The most recent sessions involved the characters happening upon a derelict cloud giant skycastle that by the machinations of fate was tied in to one of the PCs’ backstories– none of which is in the adventure-as-written. This particular side-quest kinda floated in the liminal space between sandbox and railroad, in that I was pretty confident that when confronted with an unexpected floating island, the PCs would want to check it out… but they also had the option, and the story would not have been broken, if they just shrugged and said, “Meh, the Oracle told us to go to Ironslag, let’s keep going to Ironslag.”
The scenario ended with them in something of a quandary about what to do next: they’ve still got the task at Ironslag waiting for them, but I can see at least three other directions they might want to go from here, and none of them would be “wrong answers.” One of those is even still on the Storm King’s Thunder script! Another one involves stopping at a town where they’ve never been and– guess what– that town is also a mini-quest-hub that has at least three side-quests going on as well.
I don’t expect the players to do all these side-quests, and honestly I wouldn’t want them to– it would probably feel tediously grindy to go on every monster-hunt they happened across, and you could play an MMO for that. But having the quests there gives the players “breathing space” around the main plot, in order to pursue their own agendas, which is what sandbox play is really all about at the heart of it.
It’s entirely possible that the players will look at the side-quests, say “Screw that noise!” and carry on racing towards the end of Storm King’s Thunder instead. That’s fine, too! It’s entirely consistent with my GM credo to let the players buy their tickets and get on the plot railroad, if that’s what they have chosen to do.
 Actually, very few D&D games are actually “sandboxes.” What they may be is “open world,” but that’s another discussion all together. My experience is that most of the time, when people describe a D&D adventure as a sandbox, what they mean is “not a railroad.” ;P