These "make a sandbox" books aren't really what I am talking about. i want to fill the sandbox with adventures, factions, dungeons, lairs, etc.
I explain how to do that in my book. But you don't have to take my word for it. The original blog posts are still up.
Unlike a Traveller Sandbox making a Fantasy Sandbox is less straightforward. This is because Traveller at the stellar level has a uniform g...
batintheattic.blogspot.com
But to be fair my work is more about teaching folks how to fish rather than providing an assortment of fish.
If idea seeds you are looking for then I recommend
Matt Finch's Tome of Adventure Design
The Sandbox Generator series on DriveThruRPG.
The AD&D 1e DMG
Paizo Gamemastery Books
I find the best approach to take a bunch of sources mash up a series of related random tables together to produce a word salad. Then see what inspires me. This is how I got over the hump of creating the umpteenth entry for the four maps of my Majestic Fantasy Realms. I had a lot of ideas, but they only covered a 1/3 of what I needed, so I had to find a way to consistently inspire myself to create the rest of the entries.
And the key is variety, even the best of the random table generators get repetitive if rolled often enough. I also found that ChatGPT and other AI text generator get repetitive in a relatively short amount of entries. Whether AI or more the traditional procedural generation, everything gets "stuck" and starts repeating itself. Mashing multiple sources into word salads does a good job of overcoming this.
As for pregenerated content. There are plenty of $5 adventures for the OSR and 5e that can be mined. Free sources like the Harn stuff on lythia.com. I recommend Harn Pottage in particular and the Many Manors series. My own Blackmarsh and Isle of Pyade are inexpensive and can be chopped up and their content redistributed throughout your map.
But from my experience making Hexcrawl formatted settings (like the Judges Guild Wilderlands), the key to success is a systematic, organized approach. And because the work increases geometrically as the map grows larger, learning how to write just enough to leave room to slot things in for the future but still cover what you need for the current campaign. Remember, above all, doubling the size of the map means four times the work to flesh it out.
Also you want to make sure you keep up a series of summary notes that explain how various the locales tie together. Only put enough info in the actual entries themselves to know which of the summaries to look at when it comes time to use them. If there is too much in the locale itself, then it becomes a chore to use.
Hope this helps.