Survivor campaign advice

[Aleolus]

Hello all. I am working on writing up a campaign adventure for my group, and I've hit a bit of a brick wall. Once I have it sufficiently ready, I will post it on here for critiques and for other people to use if they want, but I need some help right now.

The group comes to on an ocean beach, and because of exposition discovers they are on an island. They have to survive on this island for an indeterminate period of time (survive as in Survival skill, not as in being beset by horde after horde of monsters), and they will periodically be given challenges from some unknown individual. Accomplishing those challenges will get them either helpful supplies or clues to getting off the island. If they get off, they will be rewarded, and the only way to fail is to die on the island. They will be given a handful of supplies, mostly tools to gather what else they need. I have the first two challenges decided on, involving getting set up to survive long term and investigating a shipwreck, but I'm drawing a blank for anything else.

Also, some suggestions for random encounters for the following regions at a CR of 8 or lower would be very helpful as well:
Warm aquatic (both salt and freshwater, separated)
Warm jungle/forest (haven't decided which)
Underground (man-made mine)
Warm sand beach

Mostly animals, preferably

[Strigon]

Well, you could have them fetch something from the mine you mentioned; throwing some harsh weather at them and telling them to build something that can survive it might help; finding something hidden deep in the jungle fits nicely, I think.

As for encounters, I've always loved ambushes by Monstrous Spiders and the like. It rewards caution with an easier encounter, and it's rarely just a punching contest.

[Aleolus]

Thanks for the suggestions. Building something for the weather is iffy, since the challenges are actually coming from a person in game, and I was planning on using the mine, I just am not sure how. So far I know I want this island to have a tribe of goblins (the only semi-civilization on it), a temple to a forgotten god, an ancient ruined city, and an abandoned iron mine. I know I want other features, landmarks and the like, but I don't know what yet.

Also, I'm having the group start at level 3, and if things go according to plan they will be level 6 when they escape

[Strigon]

You're posting in a D&D subsection, so I'm assuming this is D&D; depending on his power level, controlling the weather is very possible.

How about a volcano? They're a trope for a reason.
Perhaps some coves on the coastline would be nice, too.

Personally, something I would do is make an oasis of sorts; a beautiful, lush area that's somewhat enclosed, maybe with a nice waterfall.
The twist happens at night, when something truly awful awakens; maybe it's infested with ghouls, or even a handful of Wights? Imagine them waking up to their fighter being dragged away by this nightmarish figure, and then having to fight for their life as the walls that once made them feel safe locked them in with undead horrors.
Just something to consider.

Another possible challenge could be to have them hunt down a specific animal in the jungle; a unicorn, perhaps, or a normal animal with a template or two.