Here's another whacky idea I had.
Ever have a session where 2 or 3 players couldn't make it, forcing you to cancel? That happened with my Greyhawk game two weeks ago, and it's always a bummer.
I've been thinking about ways to keep playing with only a couple players that doesn't penalize that absent PCs. On the other hand, I also don't want to risk hosing the players who made it to the session by killing off their PCs because Ralph the cleric didn't show up.
Here's my idea. I'm going to couch it in terms of 4e, but I think it applies to any version of D&D.
If you're using the DDI character builder, have the players archive versions of their PCs for each level they have gained.
Plan ahead for your campaign a bit, keeping in mind at least the outline of NPCs, treasures, or whatever that's going to show up in the next adventure/dungeon level/whatever, basically whatever comes after the current stretch.
If you're short a few players, run a flash back. The present players bring their lower level PCs. Ideally, find a break in the campaign's past that would allow for some action away from the main events.
The flashback can do a few things:
1. You can incorporate hints and pointers to future events. Maybe a couple PCs out for a night of drinking have a run in with the wandering slayer that they'll face in the near future.
2. You can drop hints to treasures or enemies that might help the PCs in the current adventure. The PCs in the flashback find a weird stone covered with runes. When the normal campaign starts next week, they enter a chamber where the stone proves useful in finding a secret door that the party might otherwise have overlooked.
3. You can give out bonus XP. The PCs who showed up get a little bonus, but not so much that they shoot ahead of the rest of the party. Since the XP is for lower level encounters, it doesn't create a big gap.
You have a few restrictions that you might want to follow: it's hard to kill a PC (he's alive in the future!) and handing out big treasures is a little weird (I forgot about that +7 holy avenger I had in my backpack!). You might want to focus on skill challenges or lower level encounters (2 3rd-level PCs might face some level 3 minions, or a pair of level 2 monsters).
This framework provides an easy excuse for an adventure aimed at only 2 PCs without messing up the current adventure. However, the flashback still advances the game. The players get to the play the campaign without messing with the campaign's pacing or plot.