Faolyn
(she/her)
Adventures, I believe, have something akin to a plot, or at least a specific direction you need to go in.?
What is the difference between "adventure" and "situation"? That's the confusion on my part. When I prep an adventure, say my current Spelljammer campaign. They party has landed on an ice planet and are exploring. I have X number of locations they can explore. There is something of a story, although it's mostly background stuff and has little or nothing to do with what's immediately in front of them, just some background stuff that might come up later.
Now, to do this, I prepared the following locations:
1. Landing location where a mud monster tries to eat them.
2. Traveling locations - 3 or so for mostly random encounters
3. A snow tunnel maze created by yak-lemmings (don't ask) inhabited by a few other bits and bobs with maybe 7 or so encounters.
4. A field of magic crystals that are the reason the party is on the planet.
5. The home of a ship wrecked gnome who has been on the planet for a while and can fill in the blanks for the players.
6. A crashed Gift Bombard with a big cannon which the party used to destroy the psychic cloud monster that forced their landing in the first place.
7. Ice station Zebra - a ruin inhabited by a bunch of stuff where the answers to some of the lingering questions about the planet lie.
Now, there was no real "story" here in this adventure. Most of the encounters could have been done in any order, with a couple of exceptions. Is this what you would call "on the fly"? I sure don't. That's a very prep heavy adventure that took me rather more time than I like to get ready. There was virtually nothing "on the fly" about it, other than maybe one of the random encounters where I tossed it in at the last minute using the pre-prepped battle map and the baddies that I had prepped earlier.
So, what do you consider to be "on the fly"?
The party hears a rumor/gets an order to go to the landing location. After exploring, they learn they need magic crystals for whatever reason. While searching for the crystals, they meet the gnome, who can tell them of the reason they crashed--and it's because of dark intelligence inside of Ice Station Zebra. The dark intelligence is the BBEG, and once the party deals with it, the adventure is over. Between meeting the gnome and reaching the dark intelligence, there's other stuff, but in all honesty, that's mostly so they can level up. There may also be elements such as "if the party gets to the field of magic crystals by week 1, they find X; if they get there by week 2, X has been moved by yak-lemmings.
What you're describing, however, is a sandbox, which are notoriously prep-heavy simply because you need to know what's at each location.