- The meteor that landed in the bay either released or mutated some worms, that have infected the sea-life and some humans. This could play out like the X-Files movie where they are trying to secretly infect everyone, or could just make them into zombies, or could mutate them. I would probably run it like zombies or mutants if it was a one-shot, and secret infection if you wanted to center that threat. The scale of the infestation is up to you as well, maybe the worms can't actually replicate so will fester out on their own or maybe it will slowly end the world. Whatever.
- The tower contains the evil chest that will turn your party into chaos cultists. My players love being chaotic evil and love chaos mutations so they would be into this, but maybe your players are likewise oriented. If you want to give them fair warnings you can let them hear the story of how the woods became evil, maybe it is known in town. Also, the corrupted priest on the road might give them a glimpse of what they could become if the gaze upon the chest.
- The changelings are barely a threat. They are just curious about the world and are learning from the human children. I mean, I guess you could make them more of a threat by having the human children be replaced by changelings or something. Damn that actually sounds fun, I should've written that into the actual one-pager. Anyways, maybe you can get one of them to become your talking familiar or maybe their livers, when dried, can be made into shape-changing elixirs. Play and find out.
Last session I offered my players three paths out of the Flatstone Fortress. One was to an abandoned cave, so I wanted to go ahead and prep in case they decide to go there next session. There is a bounty for the Calytaur, so the players are aware that they are in the region, but not that they are using the cave.