Cool stuff, thanks everyone for your ideas and suggestions - it's genuinely very helpful to have you guys to bounce ideas off of
Taking what TTW has said re. scaling the fights appropriately, I've had a think through this little idea. See what you think (comments & more suggestions are very welcome).
I want to start them out with nothing, the idea being that their 'treasure' will be building up their starting inventories from scratch (which is probably more interesting than just picking stuff off a list) and ending with enough money to buy the rest (and a little more) after the adventure ends. I might allow a thief to keep the tools of their trade if the player can justify where they've hidden it, but no weapons, armour, spellbooks etc.
If there's a personal item they desperately want back, they'll find it on the ship, but they need to look!
My current idea is as follows:
Escape a slave ship.
PCs have been kidnapped by an organised criminal gang who specialise in the illegal trade of healthy, skilled humanoids to VERY wealthy clients. Initially placed under a powerful sleep spell, they awaken now with their hands tied, caged below decks in a ship. It is completely dark to those who don't have low-light vision, and all they can tell is that they are in a moving wooden vehicle.
PART ONE: Escape their holding cages
The PCs hands are tied with poor-quality hemp rope, but they are also trapped in individual locked, steel cages. They can escape however they see fit (lockpicking, brute strength (rusty bolts), intelligence check to figure out how the hinges fit together, persuading / bribing / robbing the guard who feeds them etc.). There are various wild animals / small monsters in the cages but none are particularly well cared for and several are dead. One cage contains an NPC (bugbear or similar monster type) who will explain that they are on a slave ship and being taken for sale in the [SOMETHING] Islands. The NPC's people have been targeted by raiders for years because no-one cares about monsters so he knows a few things about them. If they attempt to kill him, he'll yell this information at them while they fight him as well as how stupid they all are etc. etc. so they will find it out no matter what they do. He will help them if they promise to allow him to go his own way once they reach land.
PART TWO: Escape the slavers
Again, there are LOTs of options depending on what they want to do so here's the layout of the ship: The slave cages are on the second deck down. Below their room in the bottom deck is a storeroom containing various boring trade items (barrels of salt, bolts of sailcloth, crates of metal ore etc.) and a few more useful items from the usual inventory list (hemp rope, water bottles, spare bedrolls, oilskin clothing etc.). Above them, two sailors are playing dice (can be seen through the deck hatch) in a small area between the crew quarters and weapon store (locked; DC 20-25), and the PCs will need to get past them somehow to explore this area and get up onto the top deck.
The deck level is most dangerous where there is the captains quarters, pilot, a few crew on watch and the ships gallery in the forward compartment. However, no-one is expecting escape, so no-one is particularly alert to events on board. Listening to the crew's discussions (on any deck), they are concerned about sailing this close to a dangerous coast where the Navy have been clamping down on slavers and pirates recently. They will also mention how the [Sea Imp Things] are out tonight, implying danger in the water.
I envision a few tactics for escape, and have various ways to deal with them:
- Jump & Swim: The simplest, but most dangerous method. They are a long way from the coast so the swim will be long and they will be hindered by any gear they've stolen from the ship. Since this could be a potentially boring end to the adventure, the swim will need to involve some small encounter with a group of [Small Imp Things] which should pose a decent challenge to the group. If there aren't any suitable D&D monsters already, I'll make something up.
- Steal lifeboat: They'll need to avoid the deck crews to get to the lifeboats and then get it off the ship which will be much more difficult than just jumping over the side. At least one crewman will jump into the boat to attack them and others will open fire as they flee.
- Signal for help Royal Navy: The PCs know that this area is heavily patrolled (hence the subdued crew / lighting) and may attempt to signal land for help by setting fires / magical spell (must be better than a caltrop) / making a racket. This won't happen by accident (e.g. as a result of attempting a coup). Since this could also potentially be quite dull depending on the player actions, the signalling will result in a naval chase which will force the ship onto a reef. The resulting battle will result in a couple of skirmishes with the slaver crew and with the naval forces (if the PCs are armed) prior to their escape into the forest.
- Kill the crew & take the ship: Yo ho ho!! Piracy is fun! This method will likely involve looting the armory, murdering the crew below decks, killing the deck watch and then facing the captain (solo boss). They'll earn a bunch of treasure from this method and a ship (which may well segue nicely into the next adventure being a 'you wake up shipwrecked...' style since I'll be damned if they all take skills in seafaring before we start
)
- Sink the ship: With enough fire the ship will sink (may occur accidentally or deliberately). Depending on what state the PCs are in by this point, there will be more or less fighting.
On land
Unless they basically sprint upstairs and throw themselves into the sea, I don't think I need any more options for this to work but if it does end too early then it'll involve something like an alarm being raised and the slavers giving chase (the slaves are EXTREMELY valuable property). The idea is then a race to the nearest village where they can raise the alarm or to stand and fight them on the beaches.
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So what do you guys think? It needs the details re. loot / stats and I need to figure out appropriate rolls for various actions (though I'll just fudge any rules I can't remember for the sake of the game).
Both from the horrible idea you proposed and from you totaly ignoring earlier ideas. Just, don't ask for ideas again. 
