I find that if you think to hard about "scenes" the tendency is to railroad players into doing what you saw in your head. If you just layout the background concepts the adventure almost writes itself and the player's will write the scene for you.
Here's my advice:
Let's use your idea of discovering an island with an undiscovered race.
Now for an adventure like this, there are several things that you can prepare ahead of time.
*A Map
*NPC stats
*Monster stats
*Terrain features
*Wandering Monster Tables
In this case, the adventure will primarily take place on an island, so your map should consist of an island. Also possibly a map showing it's position in relationship to other world features, though this is not essential.
With the map, use a key to designate where important terrain features are.
Also you might want to think about whether or not some feature of the island has kept this race hidden for so long and adjust the terrain accordingly...
Now lets think about a few "fillers"
Are there monsters on the island? (Of course there are!
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)
) If so where are they?
Are there Other NPCs on the island if so where?
Ok now that we have a map of the island and come up with some fillers let's pretend we decide several things:
*The Island is mostly jungle, and swamps, but surrounded by huge mountains and cliffs.
*The "lost" race is living in an ancient city hidden deep within the jungles.
*There is another tribe living on the coast on the beach.
*The swamps are filled with nasty critters, swarms, and maybe a big baddie like a black dragon.
*The only way to get into the center of this remote island is to either, climb dangerously slick, and high cliffs, traverse through a cave system that leads through the mountains and out the other side, or some other means you haven't thought of that your players will probably come up with.
Ok, soooo let's take a moment and mark a few things on our map shall we?
*Where is the lost race?
*Where is the coastal NPC tribe?
*Where is the cave system?
*Where is the Black Dragon's lair?
Got all that down? Good. Now no matter where your players go, you'll always know where they are at least in relationship to everything else.
Ok now.
Let's get to the next step. Now that we know what's around and where it is, let's get some important stats you'll need.
Do things one at a time.
Let's start with the NPC tribe on the coast.
First drawl a map of their Camp. Then, ask yourself some more questions:
*What is their tribe like?
*What are they likely to be doing during any given part of the day?
*Who are the major personalities?
*What do they know?
*What can the PC's find there?
*Is this a place where PCs can retreat if they are loosing?
Ok so let's say we decide the tribe is for the most part a helpful, friendly, human tribe, but somewhat suspicious of outsiders, of whom they don't see very many.
At this point list a few ideas of how they can overcome this suspicion, or even if they have to.
List stats for some major NPCs. I can think of a few:
*The Chief
*Some generic warriors
*The tribal "hero"
*The one guy who for some reason takes an instant dislike to the PCs and is therefore against you.
*The Shaman.
*Some commoners.
Decide who knows what at this point and how to get to that info.
Let's be easy. The tribe itself has stories passed down by the shamans telling of an ancient tribe living in the center of the island. Long ago a great evil came and the two races lost contact with each other.
The only way to get the above info is to befriend the tribe. The only way to befriend the tribe is to challenge and beat the "hero" in a friendly sparring match.
There is a small boy who knows of the cave system. He's not sure where it leads but found it one day while out exploring. He got scarred after he found an area in the cave littered with bones... He'd be willing to show it to the PCs if they are friendly to him.
Remember there's also the grumpy guy who doesn't like you... Perhaps here is a chance for the diplomatic PCs in the party to shine as they attempt to win over the chief.
Next... The cave system.
Drawl the map (Get the pattern here?)
Decide what's in the cave.
*Monsters?
*Treasure?
*Dangers?
Go nuts have fun. This is classic dungeon crawling right here.
Make a key that describes what each area looks like, and where important things are. (Like the island map but at a smaller scale with more info)
The Black Dragon.
Well earlier we decided where the lair was sooo:
Let's say it's lair is an old abandoned temple in the northern part of the swamp, Just a few miles away from the mouth of the cave.
*Drawl the map of the lair.
*Get the stats for a black dragon.
*List where the dragon is likely to be at any given time of the day. This way you know how likely the PCs are to be spotted wherever and whenever they go somewhere.
The Hidden Tribe.
Design them just like you did the other NPC tribe.
Look we're almost done!
Next make a wandering monster list if you want them. Just think of the types that might inhabit a lost jungle or swamp, and put them on a list. Check out the DMG and MM for more info on this.
*Dire crocodiles?
*Dinosaurs?
*Swarms?
*Leeches?
*Dire apes?
*Regular Animals?
(Also remember this doesn't always have to be monsters, swamps and jungles are dangerous places filled with things like quicksand and diseases... just think up any fitting way to kill or maim a poor PC and voila!)
Ok Now
You might want to study and make a list of all the modifiers or games rules that will come into play in a jungle or swamp... For instance:
*list the chances the PCs will have of catching a disease.
*The effects of the heat on the PCs.
*Anything you can think of to help keep yourself prepared.
Finally!
The last question to ask yourself is why the PCs would want to be here, or how they end up here. Maybe:
*They find an ancient treasure map showing the island and something about a diamond the size of a man's head...
*Or maybe their ship is caught in a storm and wrecks on the coast near the first NPC tribe...
*Anything you can think of that might capture the interests of the PCs.
Wooohoooo!!!!
BAM! Just add PCs.
Sound like a lot of work? Sure is. Some of it goes quickly, some takes more time. After a while you learn you can "wing" certain things, and if you're organized use NPC stats from one adventure for others... This works well for things like common warriors and people... This cuts down on a lot of the work.
But after all is said and done it's fun to watch the PCs as they explore this thing you've created. And if you've taken eh time to get together all of that info you can be ready for just about ANYTHING the PCs decide to do.
Hope this helps. Feel free to ask any other questions if something is unclear!
{edited some speeeeeling mistakes and UBB code stuff}