Conclusion
Of course, you could just come clean and admit you've got nothing ready. Some might say that honesty is the best policy. However, remember Rule One: The GM is a Bastard. Admitting you're out of material could be interpreted as a sign of weakness, and the players, scheming little urchins that they are, might take that as a cue to try to pull any number of stunts. If you don't have anything for them, fake it. Throw obstacles in their path. It doesn't matter if, ultimately, the mystery's as hollow as a New Labour promise. It's a delaying tactic while you try to get your brain in gear.
Like stories, RPGs have to balance plot with characterisation. Excessive plotting makes everything seem forced and makes the characters seem largely irrelevant. Basing the adventure solely on the characters can be fun but overindulgent: they need an object, a task to perform. It's a difficult balance to strike, and you can't expect each and every session to have the optimum amount of plot and roleplay. Some days you might want to get on with things and advance the story, and other times you and the players might need a rest and the chance to enjoy playing their characters. Treat each session as having its own set of goals. If you're short on material, treat that day's session as a 'fallow period' and let the players get on with, well, playing. Open the floor to them: let them make plans, interact, explore. They may end up bouncing ideas off you, and if nothing else you can amuse yourself by making their schemes go horribly wrong. Ultimately what you're doing with characterisation sessions is picking your players' brains. If they have any ideas worth nicking, do so. In a way you're giving them what they want but (and this is a very important 'but') not exactly how they want it.
If the PCs seem determined to do something with only minimal prompting from you, count yourself lucky that for one session at least you don't have to worry about The Plot And Those Bastards Who Keep Trying To Ruin It For You. Since you have an exit clause of sorts, you have a little breathing space; they can't break your campaign since the adventure isn't necessarily a part of it.
Then again, who knows? The players may find something to do that is actually rather fun and worth carrying into future sessions. If that's the case, consider yourself lucky. They've just saved your gaming session from becoming a complete cop-out. Just be grateful that for once you don't have to do all the work!