Just Wing It.

Who needs creativity?

Draw inspiration from television. You know those imaginitive series, the ones just bursting at the seams with creativity and originality? Pay them no heed. You're out of ideas: creativity has failed you and you're going to have to bluff your way through the session. You need to learn from the syndicated series -- the ones with enormous franchises. The series that run for several episodes per season and have at least seven seasons, a bunch of spin-offs. Those series have a great deal to teach us when it comes to refereeing from the seats of our collective pants.

1. Special Guest Star: Recurring characters. You gotta love 'em. Bring back a minor villain if any have survived. Have him develop an irrational grudge. Any excuse for a bit of banter. A special guest character can go quite a long way towards covering up the plot holes.
2. Do You Remember When…? Hurrah for the clip show! A chance for us to relive all those wonderful moments of a past series, and explore the character's reaction to the past events in his or her life. Or is it just a way of filling up an hour when the programme's near the end of its budget for this season? You decide! Putting their characters through past gaming sessions is a wonderful way of avoiding having to come up with anything new. Be sure to contrive a pretext for doing this, though: time travel is one way. Perhaps the characters could be on trial for past misdeeds, or maybe a villain is imitating one of the heroes, and only the good guys with their memory of what happened can convince the public as to who is the genuine article.
3. Crossover. If your unprepared session is going to have little or no bearing on the rest of the campaign, and the whole event is going to be neatly encapsulated, you can afford to put the PCs in almost any situation. Put them on another world or in another universe, let the characters muck about with a different set of toys, and put them back in the original playpen at the end of the session. Explain it away as a dream, mass hallucination, or a bit of unlikely deus ex machina. Just don't make a habit of it or chances are everyone will lose interest.
4. Back to Square One. What keeps a sitcom going for years and years? The complete lack of progress, that's what. Make sure the PCs are in pretty much the same state when they leave the session as when they entered it. If they get a super-weapon, make sure they lose it. If they're permanently injured or killed, make sure there's some kind of exit clause. If you want to carry on with the regular campaign next session -- when you've actually done your prep work -- you don't want the characters to be changed too drastically unless you really want to see how resilient your campaign is or how adaptable you are.

Of course, ultimately this approach is pretty unsatisfying. You can't expect the formula to carry you through all the way. Knowing how to be uncreative can come in handy if you need to coast for a short while, but sooner or later the need for ingenuity is going to rear its ugly head.